Relationship definitions are associations between interface definitions. They identify two specific objects that fulfill the roles on each end of the relationship.
All relationship definitions in the schema are of class RelDef, which is part of the meta schema. A relationship definition has two ends, End1 and End2. The positive direction for navigating this relationship is from End1 to End2. The reverse direction is from End2 to End1. A relationship can only exist between objects that support (realize) the interface definitions at each end of the relationship definition.
Relationship definitions also define cardinalities on each end of the relationship. Cardinalities provide the constraints for relationships by defining the minimum and maximum numbers of participants at each end of a relationship. For example, in the FDWAssetLocation relationship there can be zero to many (0..*) objects that have the role of Asset but only zero or one (0..1) object that can have the role of Location in each instance of the relationship.
Link attributes on relationships
Relationships can also have link attributes or properties. Link attributes are defined by relating link interfaces to the relationship definition when you create the relationship definition. Because interfaces expose properties, you can use interfaces to define the properties that you want on or about the relationship definition.
For example, if you create a relationship between a company and employee, you could use the IEmployee interface to expose link attributes (properties) like employee number, salary, office number, and e-mail address on the relationship.