Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 301

paint tools in the image, changing to Quick Mask mode. Thereafter, create a
selection out of the mask and from there a new layer mask.
Figure 3.114
The mask in the image
marks the area that
is to be hidden. Paint
tools can be used to
edit the mask. In the
Layers dialog on the
right, you can see a
second preview image
next to the details-
highlights layer. This
indicates the layer
mask, that has already
been inserted into this
layer.
Figure 3.115
The mask for the sec-
ond layer, median