how does the whole "interpreting ones power" thing work ? what limits the title of the hero ? how do they know there limitations ?

You’re a God when you Ascend. There’s little limits other than restrictions Paradox Space may give- For example, Roxy can create things, straight up make matter out of nothing. But she struggle with things that are not Voidy, even though Focus and Proximity to an Aspect Beacon lets her make other things, and she uses the cubes creatively to fight. John’s Windy Manipulation is able to swirl through an entire planet moments after Ascension. You truly DO get potentially Planet-Shattering, Reality-Bending powers. Even the less flashy Classpects, like Rose, Rose still gets powerful Magics and insight that gives her an upper hand in battle. This is not even going into Fraymotif Combinations.

So there’s. Really few limits other than a few logical things regarding your current situation (John couldn’t poof into wind when submerged under thick oil), or lack of concentration.