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 - documents associated with the use case giving 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