... According to "What Games Are": 'Beta' is a standard term to denote a milestone release during production in which game functionality is included and optimised (but may have bugs), game content is finished (but may have some implementation errors), and which is considered nearly complete.
In short: A beta is the finished game -or in this case an update, only thing missing is bug-fixing.
The meaning of Beta has changed over the years, and the waters have been muddied even further with terms such as Early Access.
I was taught:
Alpha: Functional build that lacks game-mechanics/features often uses placeholder assets.
Beta: All gameplay mechanics/features/assets implemented, optimization and bug crushing still needed.
RC (Release Candidate): Build that undergoes full QA testing, unless major bugs are found will become GA.
GA (General Availability) i.e. Final: Public Release.
from a technical standpoint a lot of AVNs are in a perpetual state of Beta but there are ones out there that do a somewhat more traditional release schedule by doing episodic releases that provide early builds (Beta) to some if not all supporters before wide spread release.
Edit: this was before patching afterwards was a widespread practice; in my opinion this practice seems to have dropped the GA altogether and made the players/customers the QA.