Functional and Non-Functional Requirements

Not all requirements are the same

In development or tendering projects, requirements management forms the basis for almost every decision, both when selecting a suitable system and during its subsequent implementation. But not all requirements are the same: while some describe what a system should do, others specify how well it must do it.

Before we talk about different types of requirements, we should first clarify what a requirement actually is. A requirement describes an expected behavior, property, or condition that a system, product, or process must fulfill in order to meet the needs of users, organizations, or regulatory requirements. Requirements arise from specific workflows or from overarching quality objectives such as security, user-friendliness, or performance.

Although the division into functional and non-functional requirements is widespread, in practice it is often applied in a vague manner. Much more important than a formal classification is the awareness that requirements describe both the what of a solution and the how well. This consideration helps to fully capture expectations, question them in a targeted manner, and thus create a solid basis for implementation.

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Functional requirements: What should the system do?

Functional requirements describe the specific services that a system should provide. They answer the question: What should the system do in which situation for whom and with what goal? They are typically based on the work processes of future users.

Examples of precisely formulated functional requirements:

• “The system should enable registered users to upload PDF applications via a web form.”
• “After successfully submitting an application form, the system should send the sender a confirmation email with an individual transaction number.”
• “As soon as a new item master record is created in the ERP system, it should be automatically transferred to the warehouse management software and displayed there in the inventory list.”

Functional requirements describe clearly observable processes or system responses. Well-formulated functional requirements are usually directly linked to use cases, process models, and test cases.

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Non-functional requirements: How well should the system perform?

Non-functional requirements describe the qualitative characteristics of a system, i.e., how reliably, securely, quickly, or usably it should provide its functions. While functional requirements define the what, non-functional requirements describe the how well.

Examples of precisely formulated non-functional requirements:

• “The system must deliver results within a maximum of one second for search queries with a data volume of up to 10,000 entries.”
• “All stored passwords must be encrypted using the SHA-256 algorithm.”
• “New users should be able to independently complete a full application within a maximum of ten minutes, without external help or training.”

Such requirements make quality verifiable and form an essential basis for architecture and design decisions, such as the selection of suitable technologies, security mechanisms, or operating concepts.

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Why structuring is so important during the survey process

Structured consideration of different types of requirements helps to capture requirements more systematically and comprehensively during the survey process. Qualitative expectations in particular are easily overlooked or only vaguely formulated in everyday project work. A clear structure based on relevant quality characteristics such as performance, security, or usability helps to identify potential gaps:

• Which requirements have already been addressed?
• Which aspects are still missing?
• Where are there conflicting goals?

The specialist departments also benefit: Instead of the general question “What is important to you?”, targeted prompts on topics such as response times, user-friendliness, or end devices enable concrete and verifiable answers. This creates an early common understanding of what the solution should achieve and under what conditions.

Structuring non-functional requirements in a meaningful way

Since the term “non-functional” is often too abstract in practice, it is advisable to structure qualitative requirements based on clear quality characteristics:

• Performance: response times, throughput, processing speed
• Security: access control, data encryption, attack protection
• Usability: comprehensibility, navigation, fault tolerance
• Availability: Uptime, restart times
• Maintainability: Documentation, modularity, logging
• Reliability: Error rates, system stability
• Scalability: Load distribution, capacity expansion
• Portability: Supported devices, operating systems, or browsers

This structure makes qualitative expectations tangible and allows for a clear assignment of responsibilities and comprehensible prioritization.

How to implement the structure in practice

A practical approach could be as follows:

1. Analyze existing requirements: Clearly identify functional requirements (what?) and qualitative requirements (how good?). Separate mixed requirements.
2. Create a uniform documentation structure: The classification should always be clearly recognizable, regardless of the medium (Word, Excel, or tool).
3. Provide examples and templates: Typical formulations and measurement criteria help to improve the quality of the requirements.
4. Clarify responsibilities: Who provides functional content? Who defines quality characteristics?
5. Establish early discussions: Qualitative requirements should not only appear late in the project, as they influence architecture, technology decisions, and test planning from the outset.

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Structure brings quality, even when it comes to requirements

Clearly distinguishing between functional and non-functional requirements is an effective way to avoid misunderstandings and ensure the quality of solutions. It is crucial not only to collect qualitative requirements as “non-functional,” but also to structure them specifically into concrete quality characteristics.

This creates a complete, verifiable, and robust requirements profile that serves as a reliable basis for architectural decisions, tenders, test planning, and subsequent maintainability.

If you want to clearly structure, specifically test, and successfully implement requirements, reqSuite® rm provides you with optimal support: from systematic recording and quality testing to coordination with all parties involved.

Arrange a no-obligation consultation now and find out how you can take your requirements management to a new level.‍