I think they might be referring to a seized rotor head,which distributes the presssurized/metered fuel to the injector. If the rebuilder doesnt use correct terminology,you might consider a diff vendor. Those rotor heads are expensive. If you take those pumps apart, they are very simple. If the rotor is seized, you should’ve discovered that during pump removal as it probably would’ve snapped the driveshaft off inside the pump while the bolts were being removed from the top timing gear. The only other component I think they may be referring to would be the governor ring which is a service part during the rebuild anyway. Do you know if there was any water in the fuel system?