The only "real" format is "three letters", and that it should be unambiguos. It's nice if they refer to the words in the title, but not even hat is really a hard rule ("TOS"). "STD" fits that definition just perfectly. "DIS" fits that as well (because we're just talking about shows, not companies). "DSC" is the only one that I think doesn't really do that, because it's really only one letter (aka one typo) away from DS9 or DSN - and since that is already a little ambiguous - the only constant being "DS" - I don't want another "DSx" to confuse the situation.
Thus I'm going to stay with "DIS".
No, the format is very clear, none use ST for star trek. They only use a short version of the show name, dropping the Star Trek from it. STD does not fit as it requires the use of Star trek, which breaks format. Furthermore, all shows named after ships or stations use a shortened version of the Ship/Station name. So STD break all the format rules for naming trek has used.
DIS fits, but it is very easy to confuse with Disney, who uses DIS and DISC is used by Discovery channel.