Doors and rooms are the building blocks of metarooms and worlds. While rooms define the open spaces within a metaroom, doors define the boundaries of those spaces. Any edge where two rooms meet consists of a door.
Doors have a permeability attribute (set by DOOR; called "door open value" and set by RMND in C2) which governs whether or not an object can pass through them.
Creatures and other agents use this attribute in combination with their own attribute (SIZE in C2, PERM in C3) to determine whether they can travel through a room boundary into a connected room.