One last word on the form employed to write the requirements description: Sometimes people use a mixture of “shall,” “must,” “will,” “might,” “could” and so on, to indicate the priority of a requirement. This practice results in semantic confusion, and we advise against it. Instead, use one consistent form for writing your requirements’ descriptions (“The product shall . . .” is the most common) and use a separate attribute of the requirement to identify its priority.
Link · 10. Functional Requirements