Yesterday it was build numbers, i.e. :20 and today it’s bug fix version release, i.e. third number .2 represents that. So version 21.09.2 is higher than 21.09:20, but version 21.09.2:10 (for example) would be higher than 21.09.2 (existing).
There were also minor versions releases in the past, like -2, e.g. 21.09.2-2, if combined with build number it would be 21.09.2-2:10 … yet 21.10 or 21.09.3 would be higher than any of those. However, I tend to no longer use those as it may be confusing.
Technically I should always use semantic versioning but I don’t always like to use semantic version like 21.09.0, so I just use 21.09 instead. And for possible bug fixes releases I just go with 21.09.1 or 21.09.2.