1 or 2 digits for Drupal core compatibility (Drupal 8, 9, 10, etc.). This convention must be followed.

1 digit for your module's major release version; for example, for 8.x-1.* use 1, for 8.x-2.* use 2, for Core 8.0.x use 0, and for Core 8.1.x use 1. This convention is optional but suggested for clarity.

2 digits for sequential counting, starting with 01. Note that the x000 number can never be used: the lowest update number that will be recognized and run for major version x is x001.