System functional requirements is a laundry list of all the requirements and features of the proposed system. It conveys that all functions and features are equally important. The reality is that the features that requires minimal coding are taken up first. Features that require a lot of coding or that cannot be achieved using the current API are also delayed.
There is much written about feature prioritization - Coad proposes a technique of ranking features according to customer satisfaction if the feature is included. Moreover, ranking them by customer dissatisfaction if the feature were ommitted. This will give a clear ranking of the features.
On the other hand, the feature list is the first step for a Usability Specialist. They need to order the list to rank each feature. Each feature is ranked according to the functional necessity and frequency of use. This ranking can be achieved after the task analysis process. The ranking is required to understand where the Usability team should concentrate on - the 80/20 rule.