The Mask Brush is used to edit an existing mask. You don‘t have to turn on the mode "Edit Mask" (see chapter 4.5.2.4) in order to do this.
The Mask Brush has two subtools. The brush
allows editing parts of an
image and is adding them to the selection this way. The eraser
protects
image parts from editing and removes them from the selection. You can
access the eraser quickly by holding Alt. You can use the right mouse
button, too, if you have configured PhotoLine that way (see chapter
7.10.1.20 Clear with right mouse button).