Thank you for answering! Canvas doesn't work for my needs. Spot render route is exactly what I what to avoid.
What I am asking about is:
I have a scene, say, a forest with a lot of objects like trees, grass, etc. My character, for example, is dancing in the right lower quadrant. Camera doesn't move. I want to render the whole scene 1920 x 1080 without my character just one time, and then render 200 frames of the right lower quadrant 400x400.