Its basically the possibilities in the multiverse. So C  - 137 would mean the possibility 137 of the C Possibility, so its possibility inside a possibility, A  - 137 would have different origins of Rick and the possibilities of it, whereas C  - 136 would be very close off to C  - 137, 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, 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 skipped a beat. Imagine how rick from Z - 137 would be

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.