PO destination field

      I am looking at the purchase order header and detail records a field call 
destination.   Our user says they only type it on the header screen when 
entering a PO.   Yet sometimes this destination field is also copied down to 
the detail lines - which is what we want,  but in other cases it is not.   Does 
anyone know when or how it does get copied?   I am looking at the programs and 
do not see how this is happening,  yet some of these orders have 30 lines on 
them and so I know they are not going into every line and re-typing this in.

Thanks!!