Concepts: Requirements
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.
The "+" in FURPS+ reminds you to include such requirements as:
(See also [IEEE Std 610.12.1990].)
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. Although some of these may be captured in use cases, those that cannot may be specified in Supplementary Specifications. Nonfunctional requirements are those that address issues such as those described below.
A complete definition of the software requirements, use cases, and Supplementary Specifications may be packaged together to define a Software Requirements Specification (SRS) for a particular "feature" or other subsystem grouping.
Functionality 
Functional requirements may include:
- feature sets
- capabilities
- security
Usability
Usability requirements may include such subcategories as:
- human factors (see Concepts: User-Centered Design)
- 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:
- 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.
More Information
More information on this topic can be found at:
- Concepts: Requirements Management
- Concepts: Types of Requirements
- Concepts: Traceability
- Concepts: User-Centered Design
- White Paper: Applying Requirements Management with Use Cases
