A requirement is defined as "a condition or capability to which a system must conform".

There are many different kinds of requirements. One way of categorizing them is described as the FURPS+ model [GRA92], using the acronym FURPS to describe the major categories of requirements with subcategories as shown below.

• Functionality
• Usability
• Reliability
• Performance
• Supportability

The "+" in FURPS+ reminds you to include such requirements as:

• design constraints
• implementation requirements
• interface requirements
• physical requirements.

(See also [IEEE Std 610.12.1990].)

Such categories and examples of nonfunctional requirements can be used like a checklist, to ask yourself if you have a requirement in a particular category.   But how do you come up with a complete set of candidate non-functional requirements?   Some other sources include:

• For a description of a systematic approach to capturing requirements according using FURPS+, see the The Rational Edge article 'Capturing Architectural Requirements' by Peter Eeles (http://www.ibm.com/developerworks/rational/library/4706.html).  It provides a questionnaire that lists a large number of nonfunctional requirements, along with questions for determining their applicability
• The Software Engineering Institute has also created a catalog of 'general scenarios' - expressions of quality attribute requirements – which can be reused across many different kinds of systems to define quality requirements.  For more information, see http://www.sei.cmu.edu/publications/documents/01.reports/01tr014.html or Software Architecture in Practice, 2^nd Ed., by Len Bass, Paul Clements, and Rick Kazman (Addison-Wesley, 2003).

Functional requirements specify actions that a system must be able to perform, without taking physical constraints into consideration. These are often best described in a Use-Case Model and in use cases. Functional requirements thus specify the input and output behavior of a system.

Requirements that are not functional, such as the ones listed below, are sometimes called non-functional requirements. Many requirements are non-functional, and describe only attributes of the system or attributes of the system environment.  Nonfunctional requirements are those that address issues such as those described below.  

Functionality

Functional requirements may include:

• feature sets
• capabilities
• security

Usability

Usability requirements may include such subcategories as:

• human factors
• aesthetics
• consistency in the user interface
• online and context-sensitive help
• wizards and agents
• user documentation
• training materials

Reliability

Reliability requirements to be considered are:

• frequency and severity of failure
• recoverability
• predictability
• accuracy
• mean time between failure (MTBF)

Performance

A performance requirement imposes conditions on functional requirements. For example, for a given action, it may specify performance parameters for the following:

• speed
• efficiency
• availability
• accuracy
• throughput
• response time
• recovery time
• resource usage

Supportability

Supportability requirements may include:

• testability
• extensibility
• adaptability
• maintainability
• compatibility
• configurability
• serviceability
• installability
• localizability (internationalization)

Design Requirement

A design requirement, often called a design constraint, specifies or constrains the design of a system.

Implementation Requirement

An implementation requirement specifies or constrains the coding or construction of a system. Examples are:

• required standards
• implementation languages
• policies for database integrity
• resource limits
• operation environments

Interface Requirement

An interface requirement specifies:

• an external item with which a system must interact
• constraints on formats, timings, or other factors used by such an interaction

Physical Requirement

A physical requirement specifies a physical characteristic that a system must possess; for example,

• material
• shape
• size
• weight

This type of requirement can be used to represent hardware requirements, such as the physical network configurations required.