Data is selected and combined from {TAIL ~ the master data} set to produce the data subset required for {HEAD ~ position determination} 
Data is selected and combined from { TAIL ~ the master data } set to produce the data subset required for { HEAD ~ position determination }
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD