It’s the full green field. The very top, where the green field lightens in sections, I BELIEVE is where the motion sensor fades so motion won’t be detected as well. The lowest 2/3 would have the best detection.
Vertical motion (motion entering from the top or bottom) will be hit or miss. Where horizontal motion (left to right or right to left through that field) will have the best detection.
I use mine at the top of my stairs, where I have a cat tree for my kitties to use. I had to angle the camera 45 degrees so detection worked. (essentially, the middle/top of the tree was the left side, and the top of my stairs were the right side of the field). Since the detection zone works horizontally, I had to position the camera to where the motion of them coming off the stairs and up the cat tree or vice versa was a horizontal motion from the cameras perspective.
I’ll see if I can upload a screenshot or video of my setup so you can see how I got the detection to work.