Concepts:
Requirements
A requirement is a condition or capability to which a system must conform.
There are a number of different kinds of requirements. A common way of categorizing
them is described below (see also [IEEE Std 610.12.1990]).
Functionality 
A functional requirement specifies an action that a system must be able to perform,
without taking physical constraints into consideration. A functional requirement thus
specifies the input and output behavior of a system.
Functional requirements include feature sets, capabilities, and security.
Requirements that are not functional, such as the ones listed below, are sometimes
called nonfunctional 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.
Usability 
Usability covers human factors, aesthetics, and consistency in the user interface, as
well as user documentation and training materials.
Reliability 
Reliability covers frequency and severity of failure, recoverability, predictability
and accuracy.
Performance 
A performance requirement imposes conditions on functional requirements. For example,
for a given action, it specifies the performance parameter for speed, availability,
accuracy, response time, recovery time, or memory usage.
Performance covers speed, efficiency, resource usage, throughput and response time.
Supportability 
Supportability covers things like testability and maintainability.
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, and operation environments.
Interface Requirement 
An interface requirement specifies an external item with which a system must interact,
or that sets forth 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, and weight. This type of requirement can be used to
represent hardware requirements, such as the physical network configurations required.
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