The purpose of this gage is to measure the pitch diameter size of the part. It is necessary to refine the standoff (distance from the step to the part) to a more accurate measurement because the 2-turn total tolerance limit must be divided into 3 equal zones. These three zones are known as the minimum, basic, and maximum zones, and the part is referred to as a minimum part, basic part, or maximum part.
The basic zone is the linear area within 1/3 turn from the face of the part, in or out. If the plug gage is standing out from the face (or datum point) of the internal part more than 1/3 of a turn you have a minimum part because the step on the gage stops within the minimum zone. Likewise, if the plug stands in more than 1/3 of a turn you have a maximum part. The inverse applies to external threads. Knowing the type of part, you can pick the two appropriate steps of the six step gage. B/Bt for Basic, Mn/Mnt for Minimum, Mx/Mxt for Maximum. The face of the part should be between the two steps.