Modeling relations. A relation model realizes relations between concrete concepts defined in the knowledge model. Relation is a typed association class that holds references to its Members. The type definition class RelationType describes different relation kinds by its name attribute and defines Roles that members can play, and Rules that apply to the relation type. A Role can have more than one Member. The number of role members is restricted by the attribute cardinality. A Role furthermore restricts the type of its member objects by the attribute memberType (see memberType of promRole and associated memberObjects promA and promB). Rules related to a relation kind have to check that the relations are conforming to the rules (method check).