Pid is what determines your place in the game (player identification) in the 2009+ versions of RS, pid was determed by just relogging, and you were give a number from 1-2000(2000 being the best).
Example

layer 1 has higher pid>player 2, in a stake this would mean the first/last hit, in a drop party you would get the item before player 2, even training agility, you would get through the pipe if you both clicked on the same time.
Now in 2007, pid is determined by IP, the best possible IP address is a 3rd set divisible by 8 and the last set the lowest number possible. The ideal IP xx.xxx.8.0.
Hope I helped abit