PID = Player ID
Number indicates where you are in the 1-2000 players logged in, higher number has their actions executed first.
There is actually a NOTICEABLE difference. Find a mate, tell him to follow you, and walk 1 step forward. If he moves straight away with you he has a HIGH PID, if he pauses before following it means his PID is LOWER. Do the same for yourself to check your own.
Staking-wise, higher PID means you will ALWAYS get the first hit. No joke, spam clicking won't help, ALWAYS. For melee boxing it doesnt matter too much, since hits are low, but for mage boxing it can matter a LOT - same with rapier, etc etc - any time of boxing where hits are high.