With help from many different people, we have some new documentation describing the semantics of different underscored attributes in one central place.
In case there are some attributes which you didn't quite understand, this is a good place to look. In case you think you know all the attributes, you may still want to take a look, maybe there are some "top secret" attributes that you didn't know about. Of course, it may not be perfect right now, please feel free to submit a PR if you see a mistake. If you have questions or suggestions, feel free to ask/comment here.
Also, if you're adding new attributes, please document them in this doc.