Usually I receive from my students (especially during thesis supervision) the question if there is a standard way to describe the wording of a system requirements.
An interesting approach to this is IETF RFC 2119, which defines the key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL”
1. MUST This word, or the terms “REQUIRED” or “SHALL”, mean that the definition is an absolute requirement of the specification.
2. MUST NOT This phrase, or the phrase “SHALL NOT”, mean that the definition is an absolute prohibition of the specification.
3. SHOULD This word, or the adjective “RECOMMENDED”, mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
4. SHOULD NOT This phrase, or the phrase “NOT RECOMMENDED” mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.
5. MAY This word, or the adjective “OPTIONAL”, mean that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item.
For more info please refer to the RFC here