"master clone"

this usually refers to a show taped on DAT which you have as a DAT(n) version and is typically unreliable history. somehow things morph into master sources as they get traded down the line. I haven't seen it used in reference to CD sources but I don't think I'd trust it if I did.

As for "FM", "master destroyed", "dubbed from safety copy" none of those are really considered master sources. The master tape is the one recorded at the time and place of the event. If it gets destroyed or burned up, the 1st gen copy doesn't become the master, even though it's the lowest gen available. Some people do make digital transfers or analog copies directly from the actual masters and sometimes it's from a safety copy, as above. You just need to hammer out the details if it really matters to you.

Another reason why "no CD copies of bootlegs". Bootlegs are kind of widely available where as CDr or DAT direct from master source is only held by a few people. See what I mean?