It's purely up to developers how much content they want to place in Alpha or Beta, there are not fixed rules tied to this.
Meaning of Alpha and Beta also changed over time. In the past Alpha usually meant in-house testing at developers lab by company employees, while betas were usually online tests done by players around the world.
Over time, and with companies becoming smaller, standards changed to acclimate to specific company resources, this is why Alpha and Beta can involve different processes in the hands of companies with different resources.
Some accepted standard is more or less close to this:
https://en.wikipedia.org/wiki/Software_release_life_cycle
However, again, companies will different resources available will make this stages more aligned to themselves.
If we really wanted to narrow it down as much as we can, and use some middle ground, we could say that Alphas are mechanics tests (core of the game), and Betas are content (and server stress) tests.
