When milling holes, a tool with a smaller diameter than the hole is used. Depending on the number of axis in the machine, different complexities can be achieved.
The advantage of using a milling cutter is that the same tool can create several different hole diameters and holes that are not round. Spiral interpolation is when the cutter runs in a helical path down into the material to create the hole.
Hollow milling usually requires a 5-axis machine to access the cavity through the smaller opening.