Polygroups and Masking

From ZBrush Info

Masking by Polygroup

Mask By Polygroup, located in the Brush >> Automasking menu, will apply virtual masking based upon the polygroups of the tool. If you have the setting at 100 then the first selected polygroup will be the only part of the mesh that can be edited by the selected brush. If you have the slider any lower then that it will allow multiple polygroups to be edited on a given stroke but with less intensity than the first selected polygroup.

The Mask by Polygroup in action: on the left, the original model with several polygroups, on the right, only the bottom polygroup has been deformed by the brush, without affecting the other polygroups.