The well-formedness rules of the metaclass, except for multiplicity and ordering constraints that are defined in the diagram at the beginning of the package section, are defined as a (possibly empty) set of invariants for the metaclass, which must be satisfied by all instances of that metaclass for the model to be meaningful. The rules thus specify constraints over attributes and associations defined in the metamodel. Most invariant is defined by an OCL expression together with an informal explanation of the expression, but in some cases the invariant is expressed by other means (in exceptional cases with natural language). The statement ‘No additional constraints’ means that all well-formedness rules are expressed in the superclasses together with the multiplicity and type information expressed in the diagrams.