In every Repository relationship, one object participates as the origin object and the other as the destination object. Consequently, the two collections in which the relationship itself belongs are called the origin collection and the destination collection. Every origin collection conforms to a collection type called an origin collection type, and every destination collection conforms to a collection type called a destination collection type.
When you define a relationship type and its attendant collection types, you must declare one collection type as the origin. The distinction between origin and destination is important because naming collections, unique-naming collections, and sequenced collections can only be defined as origin collections.