Orientation overlays are helpful to show orientation of an image slice. Press Alt-F11 to turn the orientation overlay on or off. A letter encased in a square indicates the viewing angle, where the image is perpendicular to the patient. Other letters show image orientation. For example, H = Head, F = Foot, L = Left, R = Right, A = Anterior, and P = Posterior.
If the image orientation is at an angle, the overlay may be a dual set of letters that appear as a 3-dimensional box.