I see these in the titles of the offline installers. What do they mean?

I guess you mean the Myst games.

I never payed much attention to it when it comes to installers. Would be interesting indeed.

You'll have to ask the developer. With GOG's older naming system (eg, nameofgame_2.0.0.x), the filenames were given and standardised by GOG. With GOG's newer naming system (name of game_somethingelse_(xxxxx), it's only the last 5 digits that seem to be given by GOG (probably as internal version tracking) whilst the rest of the filename is exactly how the developer uploads it, so the letters you're asking about are probably developer specific.

They refer to the game engine recreations that are used to run the games on modern operating systems.
SVM = ScummVM
RVM = ResidualVM
ResidualVM used to be a standalone project, but it has been merged into ScummVM in 2020.

Heh, I did think of ScummVM, but discarded that idea, since I could not think of any meaning for RVM, completely forgot about ResidualVM. Good to know :)

Oh right. Forgot these were a thing for the old Myst games. The somethingelse really should have been the internal game version. lol