If the airfield does not have an ATSU?
I don't have time to look at the document at the moment but I think that it may be referring to booking in or out of an aerodrome within a control zone.
A flight plan in that instance can usually be an informal one of basic details by phone or over the radio. You will be asked your desination, endurance and POB and sometimes if you are IFR or VFR.
D.O.