Implementation constraints are modelled separately from the use case model. The logical functional requirements of a systems are modelled using use cases and use case descriptions in the form of documents associated with the use case. These give a detailed textual description of the use case flow and other logical constraints. Using use cases for the logical requirements has a number of advantages:

	It is an outside-in view and easily understood by non-technical people.
	It is more likely to be complete than a classical functionally decomposed specification.
	It directly maps into, and is traceable to, acceptance tests, user documentation and the analysis and design models