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Descriptive Requirements are usually specified after design is complete. They are sometimes called "Subsystem Requirements"
A system can be designed and implemented in many ways. Each design has a set of dependencies on sub-components or external systems. These dependencies need to be specified so that their interactions are clear and tested.
A requirement should therefore be written for each behavior that must work as described (otherwise failures can occur):
- the User can't operate the device as they expect
- a sub-component will fail to operate correctly
- a sub-component will not operate when it should
- another part of the current component will fail to operate correctly
- a future behavior will not take place when it should
- a future behavior will fail to operate correctly
- the current state of the system or configuration on storage media is incorrect and so future operations may fail to operate correctly
- the current state of the system or configuration is transmitted to a different application or platform e.g. to the cloud, to another device, a text or email is sent, etc.
For GUIs some visual and behavioral aspects of the component must work as described, otherwise failures can occur:
- in a GUI, certain colors indicate some state to the User, e.g. red means danger, yellow means warning, green means go
- a button's state may be disabled or enabled under certain conditions
- certain notes, comments or notifications should appear at certain times otherwise the User may be confused or misled
- some visual icon should be displayed at certain times otherwise the User may be confused
- swiping left means one thing, swiping right means something else
- an internal value may be displayed in a slightly different format e.g. rounded to 1 decimal
- the size of font must be visible from 2m or less
Some behaviors or visual aspects may not matter, i.e. there is no danger of incorrect behavior or incorrect side-effects if these don't work or don't work precisely as expected.
- the background color is blue-ish. The precise color or shade of blue does not matter
- whether the buttons have rounded corners
- the final shape of the device's box
- the carrying case of the device (although the overall weight may be important)