That would definitely be the holes in the shock pistons. The drill bit # is a measure of its diameter with a larger number being smaller.

To purpose of using a specific hole size is to adjust the shock's "pack." To understand what pack is you must know that there are two ways that fluids flow, laminar and turbulent. When the flow of the oil through the holes in the piston gets too fast the flow will change from laminar to turbulent. This will drastically increase the force required to move the piston through the oil. This is known as pack.

For an example:

Let's say a shock with #53 pistons and 50 weight oil has the same overall feel as a shock with #55 pistons and 40 weight oil, when felt by moving the shocks around with your hand. But if you move the shocks extremely fast the shock with the smaller piston holes and thinner oil will feel much harder. This affects the overall feel of how a vehicle handles, corners, jumps, and adsorbs bumps.