Valve Trim:

The internal parts of a valve are together referred to as a valve's trim. The trim classically involves a disk, seat, stem, and sleeves required to guide the stem. A valve's performance is determined through the disk and seat interface and the association of the disk position to the seat.

Since of the trim, basic motions and flow control are possible. Within rotational motion trim designs, a disk slides closely past the seat to generate a change in flow opening. Within linear motion trim designs, a disk lifts perpendicularly away from the seat so in which an annular orifice appears.