How much isn't documented?

[Twi-Hard]

I've read over time that there's some undocumented stuff. A big reason why I read every comment in this repo is sometimes I learn new things not explained anywhere else. How much do you think isn't documented and is anything especially worth knowing?

[mikf]

• https://github.com/mikf/gallery-dl/blob/master/gallery_dl/actions.py
  (basically performing "actions" based on logging messages)
• Adding new instances to BaseExtractor classes ([documentation] custom site support #1658)
• URL prefixes, like ytdl:, generic:, tumblr:, smugmug:, and all the BaseExtractor ones
• An understandable guide to post processors

That's all that comes to mind at the moment.
And there is of course the fact that far too many things are all over the place.

[Twi-Hard]

Thanks :)

[mikf]

Another one: parent-child categories (ed21908, 3963dbe).