The distance between the strap and the body of the bag, measured from the center of the strap to the center of the top of the bag. The drop length measurement allows you to figure out where exactly the bag will fall on your frame.
The top-to-bottom distance measured of the body of the bag.
The distance measured for the body of the bag, taken from side-seam to side-seam.
The distance of the side of the bag, measured along the bottom of the bag.