I finally figured out the explanation for the confusing Copy Backup popup option explanation in the Retrospect Mac 17 User's Guide (you're citing that version, though Joriz runs 16.6) page 121. We must detour to page 177 of the Retrospect Windows 17 UG, where it is written of Transfer Snapshot—the equivalent operation with the same options described in a slightly-different sequence:
So why did they butcher this explanation for Retrospect Mac 8 and thereafter? The reason is that the term "snapshot" was abolished in the Retrospect Mac GUI because by 2008 the term had acquired a standard CS meaning, eventually even at Apple. Starting in 1990 the Mac UG (p. 264) had defined it:
The term "active Snapshot" is not defined even in the Windows UG; it means a Snapshot that the "backup server" has given the status "active" by keeping it in a source Media Set's Catalog. As we see from Eppinizer's first paragraph quoted here up-thread, it is the single latest Snapshot if the source Media Set has grooming disabled—but is the "Groom to keep this number of backups" number of latest-going-backward-chronologically Snapshots otherwise.
So that's why the choices in the Copy Backup popup have the word "backups" as plural. I'll create a documentation Support Case asking that the august Documentation Committee put Eppinizer's definition of "active" backups/Snapshots into the UGs. But "a backup that is kept in the Catalog" sounds silly.