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.
Many people are now aware that Autodesk has released the 2017 version of its various software packages. There are many great blog posts about the enhancements in the packages and I really like some of those enhancements. Before discussing any of those 2017 enhancements, I thought I would post some links to official Autodesk pages for you to peruse.