The difficulty displayed (rookie, advanced, expert, etc.) is simply the kind of enemies you are more likely to meet. The higher the difficulty, the more likely you are to meet higher quality monsters and in their final form.
The level is the number of monsters (as shown next to it).
If you manage to beat all difficulties up to lvl 16 you unlock the 5 hell battles. These have 70, 80, 90 and 100 monsters in them which are all set to extreme difficulty (I think). There’s also one which is only 20 monsters but they’re all mythic.
That’s what we’ve been seeing recently in GoH. In the past some battles were preset, but right now they’re all randomised.