This online calculator calculates the rectangles which will be produced by intersecting one rectangle with the another. So, technically, it performs split of intersected rectangle by intersecting rectangle. One important note is that the resulting rectangles are OVERLAPPING. Or we can say that they are NOT PAIRWISE DISJOINTED.
This is the intention, because the goal is to produce rectangles to be of maximal length in each direction. This is useful in some applications, in particular, for the family of the Maximal Rectangles Algorithms1 for 2D rectangle bin packing.
A Thousand Ways to Pack the Bin - A Practical Approach to Two-Dimensional Rectangle Bin Packing by Jukka Jylänki ↩