Might it be defining the ‘critical point’ - the part of the airfield considered secure - only accessed via the formal security channels by both passengers and crew? Passenger aircraft coming from outside the CP (eg the GA apron or maintenance hangers require an enhanced security check (pretty much every nock and cranny inside and out that could be accessed and have some mischief hidden in, checked.)
As crew, if one went through security and accessed the CP but crossed the red line, you are instantly considered ‘impure’ so to speak, and cannot go back!