Tee fitting is tasked to divide the flow by branching it. Basically this branch has the same size as the main pipe which you often refer to as a straight tee, and a different size you often call the reducing tee.
Tee in fitting also has the duty to divide the flow, it is connection fitting that has branches. Usually this branch is the same size as the size of the main pipe, we call it a straight tee. Meanwhile, if different, we call it by reducing tee.
As if on the road, if we tee this analogy as a T-junction. Talking about a T-junction, does it mean that not all of it has to be upright? True, there really is a tee that is not perpendicular, it forms a 45 degree angle. We are familiar with the lateral tee, which is usually used for low pressure.