Placing a vehicle, or any larger object, in the critical area will affect the cockpit indications. If carrying out an autoland in such a situation, the plane will attempt to follow the signals and produce an irregular flight path. Pilots usually then disconnect the autopilot and hand-fly the approach, or go-around. Airfields should have within their MATS part 2 procedures which prevent vehicles, planes being within the critical area during an ILS approach, thus protecting the integrity of the ILS signals.
Outside the critical area but within the sensitive area, there will often be planes taxiing, taking off, or landing. This does not appear to upset the cockpit indications, and allows more aircraft movements.