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Functional and Nonfunctional Requirements of Software

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Details are, in many cases, a deciding factor of choosing an outsourcing company for software development. It is crucial to understand how carefully the contractor approaches the work and how attentive they are to the little things because a quality approach is key to your future project’s success. It is also worth paying attention to how the contractor collects information about the coming digital solution when communicating with you, what questions they ask, and what details they are interested in. If there are very few questions or those do not relate to the essence of the project, you should think about your future team’s professionalism.

How your outsourcing partner handles your project specification and requirements will depend on how your project gets done and what you get at the end of your interaction. Therefore, before you start looking for potential suppliers, create a project vision to facilitate project assessment and delivery roadmap. Insufficient requirements will have severe consequences for your project and software development in the future, including:

  • Misunderstandings with your service provider regarding the scope of the project;
  • Wrong choice of technology, architecture, and team management methodology;
  • Rapid change of priorities leads to additional costs, as a rule, and to the fact that you have ten half-done features that need to be supported, but which are not released;
  • Security gaps;
  • Overhead costs associated with change requests in the case of a fixed price business model;
  • Numerous products and strategic changes leading to additional payments, high staff churn, etc.

As a customer interested in software development, you can protect yourself from these kinds of issues. Make a detailed plan, take the time and requirements, and discuss them with your service provider. We recommend holding at least a couple of online meetings, as well as not giving up the discovery phase, which allows you to interview the Product Owner and other interested parties. Collect requirements from them, then conduct a study of the target audience and market and make your suggestions about what requirements are missing and which ones should be added so that the product fully meets the target audience’s expectations.

There are several types of requirements to consider when planning a software development project.

High-level requirements include:

  • Business requirements (for example, business goals, objectives, and needs);
  • User requirements (what the user needs from the product);
  • System requirements (functional and non-functional).

Low-level requirements include, but are not limited to:

  • Stakeholder Requirements (introduced to define what the parties expect from a particular solution.)
  • User Interface requirements;
  • Market Requirements (describes the characteristics that a product must have to meet all technical market requirements and have approvals.)
  • Implementation / Transition Requirements (An additional requirement group that describes what an organization needs to successfully transition from its current state to what it wants with a new product.)

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Let’s take a closer look at functional and non-functional requirements.

In reality, there is no clear line between these types of requirements. For example, user requirements regarding system security can be classified as non-functional. However, on closer examination, such a requirement can be classified as functional, that is introduced to implement a non-functional requirement for system security

What is a Functional Requirement?

A functional requirement is a list of services that the system must perform, and it should be indicated how the system reacts to specific input data, how it behaves in certain situations, etc. In some cases, it is indicated what the system should not do. Functional requirements are also referred to as functional specifications.

The functional requirements for software help define the expected behavior of the system.

Standard forms for specifying functional requirements:

  • Description of a function or object.
  • Description of the input data and their sources.
  • Description of the output with an indication of their destination.
  • An indication of what is needed to execute the function.
  • If this is a function specification, a description of the preconditions must be met before calling the function, and a description of the final condition (postconditions) that must be met after the completion of the function is required.
  • Description of side effects (if any).

What should be included in the Functional Requirements Document?

Functional requirements should include the following:

  • Detailed information on the operations performed on each screen
  • Data processing logic must be entered into the system
  • system reports or other outputs.
  • Complete information about the work processes performed by the system
  • exact instructions on who will create/modify/delete data in the system.
  • The functional document should describe how the system will comply with applicable regulatory and compliance requirements.

The functional requirements specification should be comprehensive and consistent. Complexity implies the description (definition) of all system services. Consistency means that there are no incompatible and mutually exclusive service definitions. It is challenging for large and complex systems to develop a comprehensive and consistent specification of functional requirements in practice. The reason lies partly in the complexity of the system being developed itself, and partly in an inconsistent vision of what the system should do. This inconsistency may not show up during the initial formulation of requirements – it requires a more in-depth analysis of the specification to identify it. When inconsistencies in system functions appear at any stage of the program life cycle, the system specification will need to be changed accordingly.

Okay, now that we figured out what information the functional requirements should include, let’s look at their types. The most common requirements are listed below.

  • Transaction processing
  • Business rules
  • Requirements for certification
  • Reporting requirements
  • Administrative functions
  • Authorization levels
  • Audit tracking
  • External interfaces
  • Historical data management
  • Legal and Regulatory Requirements

We think that you could already have formed an understanding of the functional requirements, and therefore we want to give a few examples.

  • The sales system should allow users to register an order.
  • Only management personnel has permission to access income data.
  • The software package must be integrated with the banking API.

Benefits of functional requirements

Here are the pros/cons of creating a typical functional requirements document:

  1. It helps you check if the application provides all the features mentioned in this application’s functional requirements.
  2. A functional requirements document helps you define the functionality of a system or one of its subsystems.
  3. The functional requirements, together with the requirements analysis, help to identify the missing needs. They help to define the expected system service and behavior clearly.
  4. It is much easier to fix bugs found during the functional requirements capture phase.
  5. Support for goals, tasks, or user actions.

Functional requirements and their specifications

According to the definition of functional requirements that we gave initially, they describe the features of the product or functions that must be implemented during product development to meet the user’s needs.

Requirements are usually written as text, especially for Agile driven projects. However, they can also be visual. The most common formats and documents are:

# 1 Software Requirements Specification Document

Functional and non-functional requirements can be formalized in a requirements specification document (SRS). The SRS contains descriptions of the features and capabilities that the product should have and limitations and assumptions. The SRS can be presented as a single document communicating functional requirements. It can serve as an accompanying function for other software documentation, such as user stories and use cases.

The SRS document should contain the following sections:

Purpose. Definitions, system overview, and background.

General description. Assumptions, constraints, business rules, and product vision.

Special requirements. System attributes, functional requirements.

The SRS should be clear to all interested parties. Templates with a visual accent can help with this to structure information and visualize it. If you mention any other document, be sure to give a link.

#2 Use cases

Use Case can be used to describe the user requirement and the requirement for the interaction of systems, and the description of the business of people and companies in real life.

In general, the Use Case can be used to describe the interaction of two or more participants with a specific goal. For instance, a request for a page by a browser (Browser-Web-server).

Each use case has three main elements:

Actors. These are users outside the system who interact with the system.

System. The system is described by functional requirements that define the intended behavior of the product.

Objectives. The goals of interaction between users and the system are designated as goals.

#3 User Story

First of all, user stories are needed in order to figure out what the end-user needs. This is how the main pains of the user are searched for and possible ways to fix them. At the next stage, each story is discussed with the customer to detail it to a level that will describe the high-level requirements for the system. After this stage, for each story, a Definition of Done appears – readiness criteria, by which it can be understood that the requirement has been met.

So, there are three stages:

  • Writing stories
  • We discuss and detail stories
  • We define the criteria for readiness

A typical user story is written like this:

As a <user type>, I want <some action> so that <some user role>.

Also, User Stories must be accompanied by eligibility criteria. This is a set of conditions that a User Story must meet to be considered complete. Using such a set gives your team an idea of ​​what tasks they are facing. This way, every time you create a new feature, you can be sure that it meets the standard your users deserve.

Acceptance Criteria can include details such as:

  • User experience
  • Impact of the current User Story on existing features
  • Key performance metric (e.g., speed)
  • What User Story Should Do

So, based on the function you are creating and its complexity, you and your team will need to figure out what the minimum feature set should be. When it comes to the complex or the most basic function of your product, you should write as much and detailed Acceptance Criteria as possible to help your team avoid confusion.

#4 Functional decomposition or WBS

The Work Breakdown Structure is a visual document showing how complex processes are broken down into simpler components. The division is made to obtain a more manageable and controlled structure. WBS also helps to get a complete picture of the project.

The decomposition process may look like this:

High Level Function ->Sub-function -> Process -> Activity

Thus, each subsequent level of decomposition sequentially details the project’s content, which makes it possible to estimate the volume of work performed, the amount of money used, and the time frame for implementation.

#5 Prototypes

With a prototype, you can visualize the project and problematic product areas for all stakeholders and teams. In turn, the team uses prototypes to demonstrate how the digital solution works and how users will interact with it.

There are two types of prototypes:

Disposable prototypes are a cheap and quick option to represent requirements visually.

Evolutionary prototypes are a more complicated option, but their advantage is that they can become an early version of a product or MVP.

Nonfunctional requirements

Non-functional requirements are not directly related to the functions performed by the system. They are related to the system’s integration properties, such as security, response time, or system size. Besides, non-functional requirements can dictate system constraints, such as I / O bandwidth, or the data formats used in the system interface.

Many non-functional requirements apply to the system as a whole, and not to its means. This means that they are more meaningful and critical than individual functional requirements. An error in a functional requirement can reduce the system; a non-functional requirement can render the system unusable.

Simultaneously, non-functional requirements may relate not only to the software system itself: some may relate to the technological process of creating software, others may contain a list of quality standards imposed on the development process.

Non-functional requirements reflect user needs; at the same time, they are based on budgetary constraints. Take into account the developer company’s organizational capabilities and the possibility of interaction of the developed system with other software and computer systems and external factors such as safety regulations, legislation on the protection of intellectual property, etc.

Classification of non-functional requirements

Let’s take a closer look at some of the non-functional requirements:

Usability. In this case, all attention is paid to the user interface and how convenient and understandable it is for the user (whether the buttons are conveniently placed, what size they are, etc.)

Reliability/Availability. What are the requirements for uptime? Can the solution work 24/7/365? Is there any support?

Scalability. Will the system withstand the rapid growth of the user audience?

Performance. Software speed? How fast is the request processing?

Maintainability. Is support provided internally, or do you need remote access to external resources?

Safety. What are the security requirements for both physical installation and cyberspace?

The best approach to writing software development requirements is defining functional requirements and then moving on to non-functional requirements to avoid confusing the team.

Why is the difference between functional and non-functional requirements essential?

The main goal of creating software is to get a product that will meet the client’s needs first of all. Functional requirements are the primary tool through which the customer can communicate their needs to the team. Without a well-written document with functional requirements that define the scope of work and what the final product should be, it isn’t easy to achieve the desired result.

If the work scope is not initially determined correctly, then in the future, this will lead to an increase in the team’s time, and therefore an increase in cost, which is undesirable.

Sometimes, after drawing up the first requirements, it turns out that the allocated budget is not enough, but thanks to the prepared document, you can reduce some functions, focusing on its main options. It is often non-functional requirements that are sought to be reduced because excess leads to an increase in cost. In contrast, insufficient non-functional requirements affect the quality of user interaction with the software product.

Knowing the difference between functional and non-functional requirements will help both the client and the supplier gain a deep understanding of their needs, leading to a better understanding of the scope of work, optimizing costs, and ultimately meeting stakeholders’ needs.

What are the best practices for functional and non-functional requirements?

It doesn’t matter how your functional and non-functional requirements are framed, they must be understandable and as simple as possible for all project participants.

It is essential to verify that the requirement wholly and accurately describes everything that you would like to receive. It is worth avoiding unnecessary information that can lead to misunderstandings or erroneous conclusions. When writing a requirements document, use “must” instead of “should”. It is essential to be consistent when using terminology and units. Avoid abstract concepts like “the site should be able to have multiple users” it is worth clarifying whether this must happen simultaneously and how many users are meant.

How to find out how well the requirement was formed? You can check it. The main thing is that the two requirements do not overlap in meaning. In the future, it will be easier for developers to implement them and then test.

It is also worth noting that the requirements should cover all scenarios, including what the system should not do. It is important not to overdo it or overestimate it. Highlight those functions useful to users because their abundance will necessarily increase costs and make it difficult to use it. Sometimes, it can help benchmark against competitors’ products to understand what will make your product stand out from others.

As you gather requirements, document the assumptions in the requirements traceability matrix so that you can later return to the person who requested the feature with any questions you may have. Specification development is often an iterative process. As you develop your requirements, make sure they are feasible and consistent with each other.

Defining requirements can be confusing. Try to make them comprehensive to non-technical stakeholders and use visuals as much as possible to back up the information.

Finally, never consider non-functional requirements unimportant, despite the name. A website that takes 30 seconds to load may meet its functional requirements but is still unusable.

To conclude

Exact functional and non-functional requirements reduce development costs as exact requirements facilitate faster teamwork on a project. The difference between functional and non-functional requirements is as follows:

Functional requirements are easy to define because a business idea drives them. They include all the features of your future project and how users interact with it.

Experience leads to non-functional requirements. To identify them, you need to analyze the product’s performance and make it convenient and useful. Thus, while functional requirements describe what the system does, non-functional requirements describe how the system works.


While functional requirements define what the system does or must not do, non-functional requirements specify how the system should do it. … Non-functional requirements define system behavior, features, and general characteristics that affect the user experience.
A non-functional requirement defines the performance attribute of a software system. Types of Non-functional requirements are Scalability Capacity, Availability, Reliability, Recoverability, Data Integrity, etc.
The main thing is that the two requirements do not overlap in meaning. In the future, it will be easier for developers to implement them and then test. It is also worth noting that the requirements should cover all scenarios, including what the system should not do. It is important not to overdo it or overestimate it. Highlight those functions useful to users because their abundance will necessarily increase costs and make it difficult to use it. Sometimes, it can help benchmark against competitors’ products to understand what will make your product stand out from others.

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