Minor Updates

[Zach Lym]

I'm trying to understand the current versioning system and how it came into
being.  AFAICT:

* Prior to version 5.2, minor versions were published as a delta of changes [1].
* Staring with 7.0, yearly releases switched to major updates to make
versions predictable [2].

However, 12.1 was released as a minor point release [3].  Is that
because it only changed a single character or is there a stricter
technical definition of minor versions somewhere?

Thank you,
-Zach Lym

[1]: https://unicode.org/versions/
[2]: http://www.unicode.org/L2/L2014/14088-release-num.txt
[3]: https://unicode.org/versions/Unicode12.1.0/