Create Polygroup From Mask in ZBrush

To create a new polygroup of any masked area, press CTRL + W. Any masked area will be turned into a new polygroup.

Take the following masked object, for example.

Pressing SHIFT + F puts us in polyframe mode and we can see only one color (one polygroup) with the darker masked area still represented as shown below...

After pressing CTRL + W, we can see in the image below that what was formerly a masked area has now become a new polygroup. This is evidenced by the fact that the new polygroup is now its own new color in the polyframe mode...

Note: Pressing CTRL + W is the same as using Tool > Polygroups > Group Masked Clear Mask

There is another technique I’ve witnessed, which is to mask an area and then select Geometry > EdgeLoop > Edgeloop Masked Border, or press CTRL + SHIFT + E. I think this method attempts to create an edge around the mask perimeter based on the EdgeLoop setting.