OCL

OCL is an add-on feature of UML, used to describe constraints, rules, specifications. Like a programming language, it is precisely defined, enough to be machine processable. Unlike a programming language, its chief purpose is to provide an accurate way of stating requirements (rather than implementations).

What are the chief benefits of using OCL? 

• The precise thinking required to write these statements in OCL exposes inconsistencies and holes in the less formal requirements documents, forcing important issues to be clarified at an early stage, rather than discovering them during programming.
• Business rules and requirements can be stated succinctly and unambiguously: readers and writers are less likely to have different ideas about what was meant.
• OCL provides a programming-language-neutral way of stating requirements, so that (for example) where there are many components or subsystems that perhaps come from different sources and yet are to be part of one integrated enterprise, we can use the OCL statements to define the rules they all should conform to.
  We don't recommend writing UML diagrams and OCL statements alone.  A proper document should consist of a narrative text, interspersed with formal material to reduce ambiguity.