6.2.2. Collection elements
Collections may contain almost any other Hibernate type, including all basic types, custom types, components, and of course, references to other entities. This is an important distinction: an object in a collection might be handled with "value" semantics (its lifecycle fully depends on the collection owner) or it might be a reference to another entity, with its own lifecycle. In the latter case, only the "link" between the two objects is considered to be state held by the collection.