Typically, a REF value comprises the object's unique identifier (OID) associated with the object table and the row identifier (ROWID) of the row object. The ROWID is used as a means to provide quick access to the object. A REF can be used for implicit joins and easy navigation.
REF may sound a lot like a foreign key. However, there are important differences. REF is more reliable and persistent than a foreign key. This is because REFs do not refer to user-changeable values. Rather, they refer to invisible, internal values such as OID and ROWID.