These numbers refer to the overall circumference or thickness of drum sticks. For instance, a 7A stick is thinner than let’s say 5A or 3A. Depending on the sound and rebond you are looking for, the thickness of the drum sticks will vary.  If you are pursuing a thick sound that’s much preferred in rock and metal music, then you want to go with a thicker stick. On the other hand, if you are a jazz or R&B player, you might want to stick to a thinner stick which is better suited for milder accents and a warm tone, such as a 3A or 4A.