The flow field plates are {HEAD ~ an assembly} of {TAIL ~ a cathode side unipolar plate} and an anode side unipolar plate 
The flow field plates are { HEAD ~ an assembly } of { TAIL ~ a cathode side unipolar plate } and an anode side unipolar plate
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH