What is difference between Thread Forming Taps: 1/4-20 UNC GH6 and 1/4-20 UNC-2B HSS Right Hand Taps?
The GH number tells you the fit of the threads. I don't know what 2B is but I imagine it is something similar.
So you can have a looser fit, or a very tight, precise fit, or you might be expecting dimensional changes after heat treatment or plating operations so the threads need to be cut oversized or undersized beforehand to compensate (same with your tap drill size).
Basically your tap drill size determines the minor diameter of the threaded hole while the tap determines the major diameter of the threaded. At least with cut threads. With formed threads the tap drill size and the tap fit need to go hand-in-hand for proper thread formation.