Backface Masking
From ZBrush Info
Backface Masking prevents the opposite side of your model from being effected while you sculpt. This is very useful for thin models like swords.
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Backface Masking Controls
BackfaceMask: When BackfaceMask is on, then areas of the model facing away from the user will not be affected by sculpting. This can be very useful when, for example, trying to sculpt one surface of a thin model such as a piece of leather. Without BackfaceMask on, any brush radius greater than the thickness of the leather would also affect the opposite side of the leather model.
Backface masking can be used for more than just this one use, however, see the popup help for Brush:BackfaceCurve for more information.
Note that this is an AUTOMATIC effect; the mask will not be shown, and it will change dynamically as the shape of the model changes.BackMaskInt: When BackfaceMask is on, this slider determines the maximum amount of masking that will be applied to auto-masked areas of the model.
BackMaskCurve: Backface masking operates by automasking vertices depending on the angle they make away from the user. The default BackMaskCurve works well for the standard case where the goal is to ensure that sculpting does not 'leak through' to the other side of a thin model. However, by changing the curve, you can affect how strongly masking is applied to vertices, as a function of how much the vertices' normals point away from the user. For example, flattening sections of this curve allows you to produce stairsteps in your sculpting, as ranges of angles will then be masked by the same amount.
