I have started to find that models make assertions and announce restrictions (see DesigningVsModeling).  Nowadays in my classes I test the students on their ability to see the false assertions and restrictions presented by the model.  Example of a false assertion, "a customer can only rent 1 tape at a time".  Example of a restriction, "we only track the latest rental of any customer or tape."  The difference is, that an assertion is non-negotiable, it is either right or wrong.  A restriction is negotiated.  --AlistairCockburn