View_camera_offset question

Dumb question alert:

The context data for the 3D viewport has an offset list for when the ‘through camera’ view is moved around using the ‘Shift’ key. For instance I move the view and the offset shows [-0.234532432, 0.1484859303223]. What is that offset in relation to? It’s definitely not the render window, as shifting the camera frame all the way to the edge only gives me around 0.3 for an offset. I know it has some relation to the camera though, since if I zoom the window out all the way I can get the offset to hit 1.0 at the edges and I can’t move it any further.

Reason I need to know this is I’m trying to finish up appleseed’s border rendering. It works just fine for everything but a shifted camera frame in interactive mode.