Board Thread:Questions and Answers/@comment-24245551-20161124025129/@comment-24245551-20161124155032

Nurdboy42 wrote: The main Marvel universe was first identified as Earth-616 in a Captain Britain comic from 1983. Other universe designations come from comics, Handbooks, or the Marvel Appendix. The numbers are mostly arbitrary.

The universes are all independent of one another but characters can cross between them through various means.

Side note: This wiki uses a system of assigning Temporary Reality Numbers to universes whose numbers haven't been officially revealed. E.g Earth-TRN576, Earth-TRN012. Thanks for the clarification!