I assume you are referring to a pressure safety valve (PSV) or a pressure relief valve. If you have a PSV connected on the exhaust side to a relief header that can have a pressure build up in then you can use a bellows type PSV to assure that the set pressure of the PSV is not changed by any pressure build up in the header You can get a better idea of this from the first link below.
To allow adjustment for different pressures. The bellows extends under pressure. It works against the resistance of a spring to open the valve at the desired pressure.
