The trim is the pretty we see in the shower. The valve is what turns on the water and and where that water comes out. A function (or outlet) is an item the water comes out. For example 2 functions could be shower head and hand shower & three function could be for a tub & shower: shower head, a tub spout and a hand shower. A diverter will divert the water flow from one function to another. Volume controls the water volume of one function. 1/2" StyleTherm trim will come with integrated 1 or 2 volume controls. And each one of these will have the option of two different valves. You will also find a 3/4" StyleTherm with seperate volume controls.