In the industry they are referred to as "safety masters". As in assuring that the nit wits in the country that I am shipping this tape to will not have the opportunity to ravage the master tape.
They are generally made from the master or a couple generations removed from that. Unfortunately, the record business has been a bit of a seat-of-the-pants enterprise and a substantial amount of the masters of our musical heritage have simply been lost, mis-filed, or severely degraded due to volitile tape formulations, or incorrect storage.
In those cases, safety masters may be the best extant masters. And worse, in some cases, all tapes have been lost.
My copy of Mosaic's "The Complete Pacific Jazz Chet Baker/Russ Freeman" is actually recorded from vinyl records played back as some of the masters were lost. Even more embarrasing, my original 10" Chet/Russ recordings are in better shape than the ones that they used to make the records. Go figgur!