From my interpretation:
Dimension Prime - Ghostbusters I-III, Casper, Video Games, IDW Comics
Dimension 11-W - Ghostbusters: The Video Game (Stylized Version), IDW Comics
Dimension 35-N - New Ghostbusters II, IDW Comics
Dimension 50-S - Ghostbusters: Sanctum of Slime, IDW Comics
Dimension 68-E - The Real Ghostbusters, Extreme Ghostbusters, IDW Comics
Dimension 68-M - Tokyo ESP, IDW Comics
Dimension 68-R - The Real Ghostbusters, IDW Comics
Dimension 68-Q - Slimer!, IDW Comics
Dimension 80-C - Ghostbusters: Answer the Call, IDW Comics
From my understanding, "68" is the animated universes. The other numbers seem quite random. It was speculated that when the OG Ghostbusters crossed the streams, it sent the concept of the Ghostbusters across the multiverse creating alternate realities. It seemed to be just a theory though.