A projector that does not have lens shift (most DLPs) have a set offset.
This means that the image coming out of the front of the projector is shot upward if the projector is on a table or down ward if mounted upside down on the ceiling.
So you have to have room above or below the screen for the projector to sit or the image will not hit the screen properly. This also means when you mount it, you have to get it just right since you have no way to move the image up or down or left or right once the projector is mounted.
If the projector has lens sift it shoots straight out and you can turn a knob to move it up and down and left and right. There are limits to now much you can adjust but it makes placement of the projector much simpler since you do not have to be as exacting or have room above the screen (think low ceiling in a basement) Some offset can require the projector to be a foot or more over the screen top..