First there is Different Build Types
There is a link to Android and /e/ versions here e/OS + Android OS version names and numbers? - #3 by aibd
and a fuller explanation here [HOWTO] Give complete /e/OS version info easily for support, answers, comparison etc
… but beta is arguably a sub-version of dev.
Beta is fairly sound in general, but at one time was thought to be “development” but perhaps requiring verification ?
Beta is revealed in the info and / or install page for your device here Smartphone Selector