Note that even if a given version is not the one with aliases setup for it, it may still be used via the full command names.
Tracks are a mechanism to have different lines of releases. Postgres might have a “9.3” and a “9.6” track for example, each with the respective patch releases. So someone might do snap install --channel=9.3/stable to pick the latest revision released in the specific track.
We have a comprehensive explanation of channels with more details.