From Creatures Wikia
Doors have their own permeability function which ranges from 0 to 100, with 0 meaning that nothing can pass through the door and 100 meaning that anything may pass through the door.
The PERM function defines the limit at which an object recognizes a door as impassible. For instance, of an object's PERM is 100 it will pass through any door which has a permeability greater than 0, and an object with a PERM of 0 will not pass through any door with a permeability less than 100.
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