Honestly, I did not checked the manual, but I tried what it does. There is not so many of the icons so it was not so hard to find out.
You visited Tool menu, not Context menu.
In my Context menu I have this:
Ad - Orientation on the image of “Island”. Let’s suppose it is image of map - then “top” means north and bottom means south. So I noticed that the selected tiles deselected from the north down to south only. When I added the selection grew upwards (northwards) like when you pour a water into a cup. But the same process but inverted was repeated when I used Shrink Selection. So it deselected from north (top) down to south (bottom).