If you are talking about pin to cg distance on an undrilled ball, it only means a measurement of the distance the pin (top of the weight block) is from the marked center of gravity on the surface of the ball.
If you are talking pin to pap (positive axis point) on a drilled ball, it is a way to control the amount of the balls flare potential that you are using, and what part of the Lane you use it on.
On the undrilled ball the shop man is talking about, it will generally be much easier to drill a ball with a 3-4 pin to cg distance with a pin up drilling. You CAN use any pin to cg distance for that, but with shorter lengths, you have to be reasonably picky about the top weight specs to make sure you don't end up with too much finger weight.
