What exactly does P4P king mean?

[Kelman]

1) Is it the best fighter, ignoring weight classes (which would be very subjective)?
2) Is it someone who can move across weight classes and win (like a multiweight champion)?
3) Is it someone who can dominates (natrually) bigger fighters?

The general consensus seems to suggest that is number 1) which kind of makes it meaningless as it's hard to gauge people in different weight classes against eachother - I mean how can you compare Demetrius Johnsons style to e.g. Brock Lesnar...?
If it's 2 or 3) which in many cases makes sense because smaller fighter who beats a bigger fighter is by definition better P4p, then people like BJ Penn (past), Franke Edgar, Vitor Belfort, Cain Velasquez, Daniel Cormier etc would score high...

None of the above.

It's whoever proves himself to be the most effective and dominant against the opposition he faces.

For example, Frank Mir would demolish Jose Aldo in a fight, but Aldo has to rank higher than him because while Mir consistently loses badly to top 5 level guys in his division (Cormier, dos Santos, Overeem, Barnett), Aldo consistently beats them convincingly (Mendes, Edgar, Lamas, Faber).

It's all about how a fighter does against the opposition he faces, and the calibre of that opposition.

Anyone who thinks it's about imaginary fights between shrunken heavyweights or giant flyweights is a hopeless moron, because to change any fighter's physical dimensions would change him completely as a fighter.