The cutting depth (ap): is the difference between the uncut and the cut surface in axial direction, it’s two-dimensional. Maximum ap is primarily limited by the insert size and machine power.
Radial depth of cut (RDOC): the distance a tool is stepping over into a workpiece.
If the radial depth of cut is less than the tool radius, the tool is only partially engaged and is making a peripheral cut.
If the radial depth of cut is equal to the tool diameter, the cutting tool is fully engaged and is making a slot cut.
In turning and boring operations, a single-point tool cuts at a depth relative to the workpiece radius. A large radial depth of cut will require a low feed rate, or else it will result in a high load on the tool and reduce the tool life. Therefore, a feature is often machined in several steps as the tool moves over the step-over distance, and makes another cut at the radial depth of cut.