There are three established Classes of Thread, designated in the Unified series by adding “A” for screws and “B” for nuts (or other internal threads) to show definite limits and tolerances.
Class lB Thread is that in which a 1 A screw can be run in readily for quick and easy assembly. The hole is classified as 1 B. The fit is 1 B Thread, and rarely used in modern metalworking.
Class 2B Thread consists of a 2A screw in a 2B hole. This 2B Thread has wide application, accommodates plating, finishes, and coating to a limited extent and, therefore, has fair tolerance allowances.
Class 3B Thread means a 3A Screw in a 3B Nut or threaded hole for applications where tolerance limits are close.