OCL is designed to provide a clear and unambigious way of describing rules about the behaviour of elements in a set of UML models. Constraints will be captured throughout the requirements and analysis phases, and translated in the design and implementation into checks on the system that is being constructed. The primary purposes of a constraint are as follows.
- they are used to specify pre-conditions and post-conditions on use cases and operations.
- they are used to describe invariants in operations.
- they are used to describe guards on transitions.
- they are used to describe invariants for classes and types in the class model
OMG, 1999.