The optional Back Up Object parameter, when present in an alarm notification, specifies the object that is providing back-up services for the object about which the notification pertains.
This parameter shall be present when the Backed-up status parameter is present and has the value "BACKED_UP".
This is consistent with the ITU-T X.733 definition.

Readable by NMS? Yes
Writeable by NMS? No
Invariant? Yes
