Fight by fight primarily. Tournaments have their own slew of rules and regulations to help when they're made.

The existing tier sytem is vauge and fuzzy, so it only serves as a way to approximate a character's power before closer inspection and comparison comes into play.

Low-tier denotes a character comparable to a well equipped human melee combatant; often stronger, but not such a degree that they can immediately steamroll a medival footsoldier or easily win against a small group.

Mid-tier is everything between low and high tiers.

High-tier starts with things like telportation, supersonic movement speed, tank-lifting strength, shrugging off gatling gun fire, punching vars across a street and ect.

Any attempts to further categorize power levels fail due to how broad the possibilities are in RP fighting, providing too many factors to reliably or conveniently take account for.