The pull area is an approximation of the length of the rivet body that will be left after you have popped the stem. This is the body of the rivet, and the portion that actually holds things together. If the body is too long the parts won't come together ... if it is too short the rivet will not pop.

They come in several lengths for each diameter.
The part number indicates size ...

RSA 1/8 ... the "S" means small and = 1/4 pull.
RMA 1/8 ... medium and 1/2 pull
RLA 1/8 ... large and 3/4 pull
etc., etc.

Other vendors probably have different coding.
Before you get ready to do the final install it would be a good idea to practice on some scrap metal. Drill 1/8" holes in a couple of pieces and pop some practice rivets. Then drill them out to get the feel of it. You will be a pro in 15 minutes.

shoeless