It's an ITU spec naming convention - think things like X.25. They're all <letter>.<digit>+. The letters aren't often very mnemonic. The letter is a large bucket classification, audiovisual and multimedia systems for H.

https://en.wikipedia.org/wiki/ITU-T

http://www.itu.int/en/ITU-T/publications/Pages/structure.asp...

Don't know, but it was associated with the ITU as opposed to the ISO before they decided to merge their efforts