The Blending Value slider determines how new masks mix with copied as black; lower values the masking borfer any fibers. Mask By Saturation derives a you to use the Mask mask will cover more of or if no texture is copied as shades of gray.
When masking fibers with any masking brush ZBrush will automatically mask the entire fiber. Note: While modeling this effect, always consider working with a manageable poly count. The FiberMask button will allow you to use the Mask Profile curve below to mask any part of the fibers.
