Masking
From ZBrush Info
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Masks
A mask is an area of your model that is 'shielded' from sculpting, painting, etc. Masks only apply when in 3D Edit mode. (See ZBrush Modes.)
Note: The sections below talk about manipulating masks directly as you model. However, the Tool:Masking submenu contains quite a few controls that can be used to generate masks automatically, affect properties of masks and mask drawing, etc.
Masked areas are not on/off. Instead, they can be unmasked, partially masked to some degree, or fully masked. The effect that operations such as sculpting have on masked areas of a model depends on the intensity of the mask at each point.
Also, see the Transpose feature for details on a specialized type of masking called topological masking that can be extremely useful when masking limbs and other model parts that correspond naturally to the flow of a model's topology.
