There are times working within Revit that Masking Regions are needed in order to hide/cover model information within a project file. There can be various reasons for this, so I won’t discuss the “why” you would do it. You will recognize the need when you confront it. However, when working with Masking Regions, it is always good to know the guidelines and rules for how they work.
Following is an illustration of a Masking Region covering part of a simple model.
This article addresses using masking regions within Revit project files. Masking Regions inside Revit families will have different characteristics.
Some guidelines for using Masking Regions in Revit:
- Masking Regions placed in a model are not carried through to other models in which it is linked.
- For instance, if an Architect places a Masking Region in the Architectural model, it will not affect the Engineer’s model.
- If you use anything other than Invisible Lines for the boundary of the masking region, the lines will be visible.
- Masking Regions can be any shape.
- Each edge of the Masking Region boundary can be a different line style.
- Masking Regions are part of the Detail Items category.
- If Detail Items are turned off in a view, the Masking Region is ineffective and the model information behind it will not be hidden.
- Annotation categories (such as text, tags, and dimensions) will NOT be hidden by masking regions. Model categories will be masked if they meet other rules.
- 3D views cannot be masked with a Masking Region, even in a locked 3D view on a sheet.
- Masking Regions should be pinned so that they are not accidentally moved.
- A Masking Region “sent to back” will still mask the model information, whether the model information is in the same model or a linked model.
- Masking Regions will mask lines, text, etc from a linked CAD file.
- If a Masking Region is placed in an area that can be seen in adjacent dependent views, it will hide the model information in both views. You will need to hide the Masking Region in the view where you desire to see the model information.
- The Masking Region command is available from the Annotate tab and the Detail panel.
Visual Style
The various visual styles that are assigned to a view will affect whether the masking region is effective.
- Wireframe – the masking region will have no effect and the masked information will be visible.
- Hidden Line – the masking region will hide model information behind the mask.
- Shaded – the masking region will hide model information behind the mask.
- Consistent Colors – the masking region will hide model information behind the mask.
- Realistic – the masking region will hide model information behind the mask.
Additional Important Information:
I have seen a situation where a masking region placed in an electrical model did not mask the model information from a linked Architectural model even with everything appropriately set. The solution to allow the masking region to work was to go to the Manage tab , then MEP Settings -> Electrical Settings. The checkmark for “Draw MEP Hidden Lines” was removed and the masking region worked correctly.
Applying Technology to Architecture 
Thanks for the thorough post. However, I was hoping to find a way to use a masking region in my MEP project file without masking the linked Arch file. The mentioned “Draw MEP Hidden Lines” didn’t work.