Which door do you mean?

The one inside the temple itself where the pupil sleeps?

In my opinion all the doors have the same purpose, despite the lack of walls each is a cut off point. The one outside the temple seperates it from the outside world whereas the ones inside the temple are to seperate the living/sleeping areas from the actual prayer area. 

You have to remember that Kim Ki-duk is not a buddhist and is only trying to interpret and understand.

So what is your interpretation?

Cheers Trev.