Functional and non-functional requirements

Software system requirements are often classified as functional requirements, nonfun<;
tional requirements or domain requirements:
I. Functional requirements These are statements of services the system should
provide, how the system should! react to particular inputs and how the system
should behave in particular sinlations. In sOMie cases, the functional requirements
may also explicitly state what the system should not do.
2. Non:functional requirements Thesc;: are constraints on the services or functions offered
by the system. They include tim~ng constraints, constraints on the development
process and standards. Non-funcltional requireme,nts often apply to the system as
a whole. They do not usually just apply to individual system features or services.
3. Domain requirements These are requirements that come from the application
domain of the system and that reflect characteristics and constraints of that domain.
They may be functional or non-functional requirements
In reality, the distinction between different types of requirements is not as clear-cut
as these simple definitions suggest. A user requirement concerned with security, say,
may appear to be a non-functional requirement. However, when developed in more
detail, this requirement may generate other requirements that are clearly functional,
such as the need to include user authentication facilities in the system.

User
requirements
-----------------
Client managers
System end-users
Client engineers
Contractor managers
System architects
------------------

System
requirements
--------------------
System end-users
Client engineers
System architects
Software developers
--------------------