The screen focal length is a parameter of the rear projection screen. The screen focal length should match the projection distance for rear projection screens to ensure a right-angle light exit of the screen to the audience. If the projection distance is shorter than the screen focal length, the light exit is diffusing (light in the upper area of the screen goes up, light from the lower area of the screen goes down). This situation should be avoided.
If the projection distance is longer than the screen focal length the light output of the screen is focussing in one point. This can be helpful with large screens in smaller rooms.