As Boni, its a difficult question to answer. Because there is not really that much to know about pivot points. Its basically just the rotational point of an object. For instant if you load a box in Poser and rotate it, it will spin around the bottom part of the box on the X and Z axis and spin around itself on the Y axis. This is because the pivot point is at the bottom center of the box.

If you open the joint editor you can see where the pivot point it placed for a given object (Green cross). Its the same for bones in a character, where the pivot point is placed at the root of the bone and aligned towards the end of the bone (called End point). So normally you would place the bone at the shoulder towards the elbow of a character which will allow the character to rotate the shoulder.