is mask by feature help?
Mask By Features
The Mask By Feature button will apply a mask to the geometry Border, PolyGroup border and/or Creased edge, depending on the options selected to the right of the button. The options can be selected however you want, either on their own or together.
Border ***8211; when turned on masking will be along the visible border of a mesh. Either the mesh must be ***8216;open***8217; or part of the mesh must be hidden for this option.To see Mask By Features > Border in action:Load the LightBox > Tools > DemoHead.ZTL.Make sure Border is turned on and click the Mask By Features button.Only the edges of any openings will be masked.Groups ***8211; when turned on masking will be along the borders of all PolyGroups.Crease - when turned on masking will be along creased edges.
half of the sphere was hidden and then click Mask By Feature:
only the visible mesh 's border get masked:


