ProcSignalSlot vs. PGPROC

Hi,

do I understand correctly that a BackendId is just an index into the 
ProcSignalSlots array and not (necessarily) the same as the index into 
ProcArrayStruct's procs?

If yes, could these be synchronized? Why is ProcSignalSlot not part of 
PGPROC at all? Both are shared memory structures per backend (plus 
NUM_AUXILIARY_PROCS). What am I missing?

Regards

Markus Wanner