The format is really [major version][train][audience]?[build number][mastering version]?, where major version is numbers, train is a capital letter, audience is a digit, build number is numbers, and mastering version is a lowercase letter (and things with question marks are optional, and of course Apple does its own thing from time to time). So the current beta for Big Sur, which is 20A4300b, has

20: Major version (happens to match the XNU kernel version)

A: Train (First “point” release, although that is not how marketing does it)

4: Audience (Developer seed)

300: Build number (300 builds since the train started)

b: Mastering version (forked from mainline two builds ago for polishing)