{HEAD ~ A p code frame} or simply frame represents {TAIL ~ a unit} of action or operation in the system 
represents
{ HEAD ~ A p code frame } or simply frame represents { TAIL ~ a unit } of action or operation in the system
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH