According to Setting Debug Log Filters, the debug levels for each category are as follows:
However, when we create a new User Trace Flag (Setup > Logs > Debug Logs > New (User Trace Flags)), using the lookup window's Quick Create button, we get the additional level of "INTERNAL" that sits right between NONE and FINEST.
Even more interesting, when we later go back to edit the Debug Level, we are shown values of NONE, but with everything (?) selected:
I've accidentally created the one named INTERNAL shown in the screenshot, as I'm usually just the one to set everything to maximum when I need to find a rather tough problem, but now I'm thinking I should delete it/set it to more appropriate levels, as it may have unintended side effects that I'm not aware of.
The main question here, though, is what does setting this value do, and could it have any harm (besides, of course, creating large debug logs)? Is this some sort of bug, or does it represent some sort of value that would be used by the platform developers?