when you got a 100 wide door, and you open it with lip=0, then it will slide to the left (or right) with 100 (eg it opens fully). If you set lip to 50, it will only slide 100-50=50 to the left, and it will open just half, or if you set it -100 then the door will go away
Example map with one door in it (and a light, and player_start) the door's lip is set to 50 (you can see, its not opens fully) wait (delay before close), is set to 2, and speed (moving speed) is 100 (only for half-life i think)