@Vortaxonus
Sublegendaries can participate in the ingame battle facilities like the Battle Frontier, Battle Subway, Battle Tree, etc. while legendaries are banned from those facilities.
For example, you can take a Pheromosa, Tapu Koko, or Suicune to the Battle Tree but you can’t take Mew, Rayquaza, or Giratina there.
Those same rules usually tend to apply to the official VGC tournaments as well minus some rare cases like the 2010 and 2016 tournaments.
As for the mythical thing, Prof.NightJack’s right. Mythical Pokemon are just a name for Legendaries that were originally obtained through events. That being said, Mythical Pokemon were on the Legendary list that I mentioned earlier (which makes sense because they are banned from the ingame battle facilities and the VGC tournaments).