Which fitting is permitted only for horizontal-to-vertical flow applications?

The sanitary tee is specifically designed for use in waste and drainage systems where horizontal-to-vertical flow applications occur. Its shape allows for the efficient transition of fluids from a horizontal pipe to a vertical pipe without creating excessive turbulence or pressure loss. This is vital in plumbing systems to promote proper flow and prevent any potential blockages.

In contrast, other fittings, such as the wye fitting, coupling, and reducer, serve different purposes. The wye fitting is versatile and can accommodate flow in multiple directions, making it suitable for both horizontal and vertical applications. Couplings are straightforward connectors for joining two sections of pipe in the same direction, and reducers are designed to transition between pipes of differing diameters. None of these fittings are specifically restricted to horizontal-to-vertical flow applications like the sanitary tee. This specialized design is what makes the sanitary tee the correct choice for this particular question.