What I mean is that you 'break' the level up in different blocks, then the engine calculates if it can 'see' block(sector?) 1 from block(sector?) 3.
Say if I make two rooms and a twisty corridor in between, then I put a sector (I'm not sure if this is what it's called) around room one, another around the corridor and yet another around the last room. Now, if I stand in room one the engine make some calculations on weather or not it can see the other sector from the one I'm in. If it can then it draws whats in it and if it don't it doesn't draw it. Thus making the map run better since it don't draw everything at all times.
Crap... I really can't explain it better....
Close the world, txEn eht nepO.