It's basically the possibilities in the multiverse.

So C-137 would mean possibility 137 of the C Possibility, so it's a possibility inside a possibility, A-137 would have different origins of Rick and the possibilities of it, whereas C-136 would be very close to C-137.

For example, in the Citadel episode, the news reporters are from D-716. Even inside that there are many possibilities, so there are D-716, D-716 B, and D-716 C, and all three have different experiences as marked by their faces, but still such a close origin that they are all news reporters.

So according to this, when Rick and Morty ruined their dimension and came to this one, they were from a very nearby possibility, as they didn't skip a beat.