Popcycle.dat controls the population density for each specific region of the map, e.g. business areas, entertainment zones, shopping zones and more. Using higher values increases the amount of pedestrians and vehicles roaming, however this might use up more memory as the game needs to store more information about the pedestrians and vehicles spawning. While it is often said that the values need to add up to 100%, it has been observed that breaking this rule will not necessarily break the game.
At least in the Enhanced & Expanded version of GTA V, the value "PercCopCars" is never used by the game's .exe.