In my understanding, this just a "tool" for measuring. If you want to know for example how many pixels a person or other object has in a specific area, you can draw a pixel counter box in the preview image.
After that, you can setup the IVS rule detection rectangle to the minimal and maximum pixels. If a person in the image counts lets say 50 x 300 pixels, and if that is the minimum you want it to detect, you can set that minimal detection box to 50 x 300 pixels. The maximum detection box, you would then set to a bigger pixel count.
It is just a measuring tool.