The projector frame shows you the entire area you can use within this track. The camera frame shows the area inside the projector frame. If the projector frame gets smaller, only this area can be used, everything outside this frame will be cut off.
Thanks Sam; with your words I'm starting to understand. I understand to how it works in your video (as well as in mine), but I remember that long ago it didn't work that way to me. https://www.youtube.com/watch?v=gcEiWQHTpmU&list=PLr12wAaNzDdVSzd8_2pLzio_D0... In the tutorial (from 10'27" to 11'20") the projector remains untouched to FullHD (the output of the project). Moving the camera, the green box remains fixed to the edges of the Compositor view while I pan and zoom the frame. The video is about a year old. Now the green box is not fixed but moves in a way consistent with the pan and zoom I do. That's the right behavior, but why wasn't it like that before? There is something wrong with the way I think.