What is this tool, and how do I stop it working?

On snagit for Mac, when I am in the image editor, I always see most of the area shaded out except for a within an area like a selection rectangle. Thus:



it isn't THE standard selection area. IF if move/resize it, the actual capture is variously moved/cropped resized.

I have no idea what it is or how to enable/disable this piece of functionality.

Any suggestions gratefully received.