The total backpressure is generated from two components; 1). superimposed backpressure and 2). the built-up backpressure.
Superimposed back pressure: the static pressure that exists on the outlet side of a closed valve.
Built-up back pressure: the additional pressure generated on the outlet side when the valve is discharging.
In a conventional design (no bellows), the superimposed backpressure will affect the opening characteristic and set value, but the combined backpressure will alter the closing (blowdown) and re-seat value