I presume that they are refering to the vacuum proportioning valve. This is the device that taylors the constant vacuum of the engines vacuum pump into a vacuum signal that resembles a gas motor (zero at full throttle to 15 inches at idles or there abouts).
I hope they diagnosed all this as there should be no reason to replace either unless something is wrong.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician