Sort each element by its relation to the frame.

Inside the frame: Photographer reflected in a shop window, Far room wall visible through a doorway

Outside the frame but signalled inside it: Sitter looking past the right-hand edge, Hand reaching in from above the top edge

Outside the frame and unsignalled: Traffic behind camera leaving no trace

The middle category is the useful one and the one beginners overlook. A shadow, a gaze or an intruding hand recruits the viewer into building what is not there, which is why photographs that use it feel larger than their edges. The last category is what "excluded" really means: no trace, no inference, no picture.