In locating the site for a defensive firing line, it must be divided into three sections : firing line, immediate support and reserves. In doing this several opposing factors should be taken into consideration and their relative importance judged...
U.S. Army Artillery and Missle School Tactics/Combined Arms Department
"This 1964 SOP is published to provide students with information necessary for the solution of School problems. In addition, it may be used as a guide for any officer designated to write a unit SOP."