Old Alfresco Community Edition versions have used the letter suffixes a, b, c... to enumerate proper (non-nightly) builds of the Community Edition. So these were meant to be sort-of stable releases that could be depended upon to deploy a system and not just be some hot code. Of course, a and b lettered versions used to be less stable than later letters, since these versions usually included new, yet untested features, that were then ironed out with feedback from the community in the next releases.
Alfresco no longer uses the letter suffixes to mark official Community Edition version releases - but they can still be found in some instances on the technical layer (e.g. Alfresco Share Maven artifacts). Generally speaking, nowadays a proper Community Edition release will have the "-ga" suffix to it to mark it as a "General Availability" release.
