Why axial and radial depth of cut have separate limits
Depth of cut is not one single number in milling; axial depth, how deep the tool cuts down into the material, and radial depth, how much of the tool diameter engages sideways, hit different limits for different reasons. Push either one too far and the results are different too: excess axial depth breaks tools at the flute limit, while excess radial depth causes deflection, heat buildup, and chatter even on a tool with plenty of flute length left to spare.
How the numbers are calculated
Maximum axial depth of cut is capped by the usable flute length, since cutting deeper than the flutes themselves means the shank or holder starts contacting the material. Maximum recommended radial depth of cut is expressed as a percentage of tool diameter, since a full-width slotting cut engages the entire tool and generates far more heat and deflection than a light side-milling pass at a small radial stepover.
Practical tips
When either number is pushed close to its limit, back off the other: a tool running near its maximum axial depth should use a lighter radial engagement, and a full-slot radial cut should generally stay well under the maximum flute length rather than combining both limits at once. Rigidity of the setup, tool material, and coolant all shift these limits in practice, so treat them as a starting point to back off from when chatter or excessive heat shows up rather than a number to push toward by default.