F, G, D, and C are trigger/hammer operation identifiers:

F = Safety/Decocker (examples are 92F and 92FS)
G = Decocker only, no safety
D = Double Action Only - usually flush hammer
C = Constant Action (aka Type C) - (new) for PX4, pre-cocked double action

L, M, and P are capacity/configuration identifiers:

L is usually a compact with a slightly shorter grip. In the case of the Cougar, it is also a lighter barrel and beveled slide.

M is a single-stack L

P (as in Cougar L Type P) is the Cougar L slide, but with a full-length magazine - The Stoeger 8000 in 9mm is identical to the Beretta Cougar L Type P

S and SB are early models of the 92 and are usually blued and have a round trigger guard.

F and FS are the more and most recent versions of the 92. ALL model 96 pistols are considered FS unless a 96D or 96G, but do not always carry the designation.

SD is a designation for "Special Duty" and will configured in a manner that would be most desirable for a duty weapon. Three examples of this are the 92G-SD, 96G-SD, and PX4-SD 45ACP.