Tees straight and reducing
The primary purpose of a Tee is to make a 90° branch from the main run of pipe. Standard there are 2 possibilities, on behalf of the equal tee and reducing tee. The equal tee (or straight tee) is used as the branch has the same diameter as the run-pipe. The reducing tee is used as the branch has a smaller diameter as the run-pipe.
Dimensions and Standards
When we talk about a tee NPS 3, a equal or straight tee is intended. With a tee NPS 3 x 2 a reducing tee is intended. Although, officially a reducing tee will be indicated by 3 diameters, namely 3 x 3 x 2 (A x B x C). The A and B measure stands for the Nominal Pipe Size of Run-Pipe, C measure stands for Nominal Pipe Size of Outlet.
This designation actually is superfluous, but comes from the time that tees were available in 3 different ends of diameters, such as
4 x 3 x 2 (A x B x C). In this implementation of course it is necessary, to specify 3 different sizes.
That kind of tees is probably no longer available. Applying a equal tee with a concentric or eccentric reducer is now the usual practice..