A mask is an area of your model that is ‘shielded’ from sculpting, painting, etc. Masked areas are not simply 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.
To create a mask, hold the Ctrl key and drag over your model. Different masking brushes can be selected from the Brush palette while holding the Ctrl key.
Hiding/Unhiding Parts of the Mesh
You can show and hide parts of your mesh to make it easier to work with. For example, you might want to work on just the head of a character. In that case it makes sense to hide the body so you can concentrate on what you are doing.
To hide parts of the mesh, hold the Shift+Ctrl keys and drag over the model. Different selection brushes can be chosen from the Brush palette.
PolyGroups allow you to organize the mesh into different colored areas. These can then be selected and shown or hidden, making work easier. To view PolyGroups, the PolyFrame button must be turned on. (Hotkey Shift+F)
To create a PolyGroup, draw a mask and then press Ctrl+W. A new PolyGroup will be created from the masked area. Another way is to hide parts of the model using the selection brushes before pressing the Group Visible button in the Tool >> PolyGroups sub-palette.
Ctrl+Shift+clicking on a PolyGroup will hide all other PolyGroups.