Reviewing the active upgrade process for Datacom/AD using an application like CA 11, CA 7, etc. and looking for a back out plan , should the upgrade fail.

The online documentation states that running the back out job AXACTDDR is "rare".

What would be the "rare" circumstance requiring to run AXACTDDR?   

Datacom/AD 14.0

Datacom/AD 15.0

Datacom/AD 15.1

Once the two active upgrade jobs , AXACTDD1 and AXACTDD2,  for 14.0 to 15.1 have been executed, the active MUF upgrade process is complete for one candidate version 14.0 MUF instance. 

The Datacom Dictionary (DD) upgrade is designed in such a way that even if the software needs to back out to the Version 14.0 level, it will run the same against a Version 15.x DD, and therefore this fallback job (AXACTDDR) is not required. At a high level, running AXACTDDR will only remove new entities created for Version 15.x, which the Version 14.0 software will not know about. There is less harm in leaving these new entities in place than in trying to fallback.

AXACTDDR is not necessary unless the Datacom/AD 15.1 Datadictionary  is absolutely destroyed or if the DD/DDD gets corrupted somehow whether it would be through bad DASD, user errors, or something unusual.