Imagine looking down on the top of someone's head. face-forward would be 12 o'clock, with the hour incriments corresponding to positions around the head. Trace down to the waist, and you'll have you're holster position. 3:00 would be on the hip under the right arm, 9:00 the left arm, etc. For me (4:00, on the hip just behind my right arm), the advantage is for concealment and comfort. I can hide the grip better if it's not sticking straight back, and I can sit comfortably in the car with it there. Some people carry a certain position for tactical reasons, or because drawing is more comfortable from there. It's mainly just personal preference.
Walther P99AS 9mm
Beretta PX4sc 9mm
Walther P99 .40 S&W
FrankenAR-15
Type II Phaser