Tolerance is important because it ensures that the internal threads created by the tap correspond to the external threads of a fastener. Tolerances 1B through 3B refer to the Unified National Thread standard for internal threads, and correspond to 1A through 3A for external threads. Threads are divided into three classes based on their tolerance (amount of size difference from exact size) and allowances (how loosely or tightly the fastener fits to the threads), with Class 1 having the largest allowance, and Class 3 the most strict. Class 2 has a fair tolerance allowance and is the thread standard used for most general purposes.
Tolerances H1 through H10 apply to taps up to 1” in diameter, and describe the specific pitch diameter of the tap. The basic pitch diameter (what is specified) may vary from the actual pitch diameter of the tap. For example, H1 is the basic diameter +0.0000” to +0.0005”, and each step up is an increase of an additional 0.0005”, so H2 is +0.0005” to +0.0010”, and H3 is +0.0010” to +0.0015”. H10 is the basic diameter plus 0.0045” to 0.0050”.
Taps with tolerances with an X, such as 6HX, are intended for thread forming, as opposed to thread cutting, taps.