A requirement is a statement of need or intent. In most cases, the statement has been formally designated by the originating authority as a criterion for selection or approval.
In legacy specification practices, these were called “required characteristics”; one way to think about them is that they’re just characteristics to which grammatical “modality” or “mood” has been attached. Grammatical moods include, but are not limited, to permission (“may”), possibility (“might”), obligation (“shall”), and normalization (“should”, “ought”)1.
In most discussion, I use the term “priority” instead of “modality” or “mood” because it is usually better received by the Engineering Staff and Management2.
Five phrasing forms were used in the legacy: shall, shall, where practicable, should, may, and will. Of those, all but the first and last are often deprecated today.
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