All frames that are currently selected will be concentrated into
one mask group. The manner in which the frames lie on top of each
other influences the result. The frame lying right at the back will be
masked by all other frames above
it. The masked frame
determines the writing mode of the mask group. If only one frame was
selected when clicking on the Mask icon, an error message will appear.
If the result of the masking does not match your expectations, you
can split up the mask group again (see Ungroup frames or masks
below) and check whether the order of the frames is correct and also
whether all the frames were selected.
Perhaps there was also a misunderstanding about the way the masking mechanism works.
