The threads on a tap have a very small of radial relief,but ot is not enough to accomplish cutting, it is there to reduce drag the relief. What does the cutting is all on the chamfer, the longer the chamfer, the thinner the chip, although, I have observed over the years that a taper tap usually has more drag than a plug tap. Taper taps such as pipe taps, have more radial relief, and cut all the way back when tapping a taper pipe thread. Acme taps have radial relief like a pipe tap, and also cut all or most of their length to achieve full depth threads.