Stable Build is annotated SB. For example, you would have 3.5-SB and this would mean version 2.1 Stable Build. These builds are stable and no hotfixes are needed.
Beta Build is annotated BB. For example, you would have 3.5-BB and this would mean version 2.1 Beta Build. These builds are beta and would probably need hotfixes.
Beta Build is annotated AB. For example, you would have 3.5-AB and this would mean version 2.1 Alpha Build. These builds are extremely unstable and a lot of hotfixes are needed.
Note that we almost never drop updates in alpha except if we are forced to.
Hotfix builds work along stable and beta builds, for example, you would have 3.5-SB-hf1 and this would mean version 2.1 Stable Build but first hotfix. If there is no number, then no more hotfixes are coming.