Another way to rank requirements is to distinguish classes of requirements as essential, conditional, and optional.
a)Essential. Implies that the software will not be acceptable unless these requirements are provided in an agreed manner.
b)Conditional.Implies that these are requirements that would enhance the software product, but would not make it unacceptable if they are absent.
c)Optional.Implies a class of functions that may or may not be worthwhile. This gives the supplier the opportunity to propose something that exceeds the SRS.