Warburton Signature trombone mouthpieces are designed as a system that facilitates the brass player. Allowing them to choose a rim diameter, cup style and depth, with a back-bore to best balance the players sound and resistance. The aim is to give the player the most efficient mouthpiece to be able perform in any given playing situation.
The Warburton mouthpieces are a two-piece system consists of fully interchangeable "TOP" and “BACK-BORE" sections. Due to the multiplicity of demands placed upon the modern brass player, the various cup designs have been chosen for ability to produce the most desired sound in any playing idiom.
In this case we are looking at the Warburton “TOP", where they have tried to give the player a variety of size both with options in rim shape/width and cup size (depth/diameter). There is a list of their top sizing to peruse.