I'll give a more thorough explnation on the version strings.
Alpha (a1, a2 etc.) indicates the map is in early development and focuses on gameplay over detail.
Beta (b1, b2 etc.) is pretty much the detail and optimization stage, sometimes layout can end up being altered in this part.
Release candidate (rc1, rc2 etc.) is the part where the map get's finished up, now usually rc gets similar improvements like beta, but rc can be considered the "final" version.