2.6.1. Using the Red "Paint Selection" Button

An image has a mode that enables you to "paint" the selection using existing tools.
In this mode colouring with black adds pixels to the selection.
And colouring with white excludes them.
To access this mode press the button with the Red frame in the lower-left corner of the image.
Note: this mode has a meta-selection, but there aren't any meta^N-selections.