All Tenor tops are to be used with the T backbore.
The S cup is a shallow "bowl" shape. The WARBURTON S-cups are the shallowest pieces on the market. For unusual upper register demands or the ALTO trombone, these models perform well.
The M cup is preferred by players doing LEAD work in big bands. These are medium depth with a modified "V" contour.
The D cup (deep) produces a very full sound.