Masking regions provide a way to obscure elements in a view. Masking regions may be useful in scenarios like the following:
You can create 2D and 3D masking regions. 2D masking regions can be created in a project and in the Family Editor when you are creating a 2D family (annotation or detail). 3D masking regions can be created in the Family Editor when you are creating a model family.
Masking regions do not participate in shading; they are always drawn the background color of the drawing area. Masking regions cannot be applied to element subcategories.
When you export a project or family that contains a masking region, any lines that intersect the masking region terminate at the masking region so that graphical intent is maintained in the DWG file.