Sometimes you want to swap one colour for another in a given area but leave the original colour in another area. This tool allows you to select an area, then select a stitch of the old colour, and finally select a replacement colour. Any stitches outside the seleced area are left alone.
This can be very useful when you are zoomed in close and want to swap a colour. instead of having to zoom out and check the colour is not used somewhere else, you can simply select the area you can see and limit the swap to that area.