Originally Posted by dabac

9 mm refers to the axle diameter where it sits in the dropout, and it's the most common one. Doesn't have any impact on q/r size.

Q/r sizes are only listed in length. Front and rear. Sometimes embellished with rear MTB (135 mm OLD) or rear road(130 mm OLD). There are a few extra long ones for bike trailer attachments, and some extra bulky ones for trainer use, but that's about it.