ILS Critical areas, iirc (not in work atm to look at the citing order), it's 35 Degrees from center of the mast (glideslope) or distribution box (localizer), and 1000 feet out. For our CATII/III sites, must be clear during CATII/III operations.
#1 reason, being reflections...also on a localizer, that's CAT II
/III. We have what's known as a Far Field Monitor, which monitors the radiated signal. Unexplainable loss in signal can cause a downgrade in category. But i think the caveat with that, is if we enter CATIII & say we lose a transmitter (Our equipment is dual transmitter, with hot standby). We can still maintain CAT-III ILS, but if weather improves, we leave CAT-III and then weather degrades again...we are not allowed to re-enter CAT-III. Assuming no qualified technician is avaliable to look at the equipment.
The day in the life of an ILS Tech, has no equal...lol