Thread classes are used to determine what type of thread is best for a particular application. Other considerations include thread form and thread series.
For unified inch threads, there are 3 thread classes describing external threads (1A, 2A, and 3A) and three for internal threads (1B, 2B, and 3B). All of these are “clearance fits” which indicates that they are assembled without interference.
Generally speaking, the higher the thread class, the tighter the fit between mating threads. For example, an assembly that mates class 1A and 1B threads will have a looser fit than an assembly that uses class 3A and 3B threads.
Classes 1A and 1B are used least often but are suited for applications that require quick assembly and disassembly. 2A and 2B are the classes used most often due to their cost, consistent performance, and ease at which they can be manufactured. Classes 3A and 3B are used when safety, strength, and extremely close tolerances are necessary. This makes them ideal for socket set screws and similar applications.