Said with "Popeye the Sailorman's" voice...
When a savy ADT user starts using Revit they can get into trouble quick when they start getting comfortable by applying the ADT concept of "Mass" to Revit's concept of "Mass". The equivalent terminology for Mass/ADT is Generic Model/Revit. When you use the Mass category for a piece of furniture you are creating trouble for yourself. Revit expects you to use the family category "furniture" for that.
What kind of trouble? You can't schedule them as furniture, you can't change them to furniture, you can't turn off massing because you "need" it visible because you made "furniture" using Massing...get the idea?
When you create a curvy swoopy wall, don't "lean" toward mass, lean toward an In-Place Wall family. Now experienced users are thinking, "But Steve, I can use Massing to create the shape I need and then apply a wall to that!"
YES, THAT is what Revit's use of the term "Mass" is intended for. Mass or Massing in Revit is meant to describe what the terms are used for in the context of architecture, building form, not arbitrary discreet pieces or parts. Revit has lots of pre-defined categories for those and for those that don't fit into one of them we have Generic Model and Specialty Equipment. As a means to describe the "broad brush" building form a Mass Element can host Walls, Floors, Roofs and Curtain Systems. Here's a quick example I did in the past that just shows massing and floor slabs, well...a helicopter and a couple cars too...
The above represents a building that is 945K+ SF, mocked up in about 15 minutes, nothing serious, just playing around.
Here is another little example I played around with on a plane ride the other night.
I was thinking of a medium size design firm in a semi urban setting where the executives would park beneath the building. The patterns are just material/surface patterns assigned to the massing. Good old shadows make it "nice"
One more for the road...this is three curtain wall systems applied to massing that describes curving surfaces in plan, elevation and section so that it has a slight "beer" belly. The massing is off in the view leaving behind just the curtain systems. I was sloppy because I didn't take care to align the curtain grids/mullions but then we did this on a short break between "real teechin"...a good old "Can Revit do X" question.
Elements created using the Mass category cannot be changed to other categories. This is because of the hard-wired behavior Revit assigns to elements using this category. If you make in-place "walls" and choose the Mass category instead of the "Wall" category you are headed for pain and re-work..."you know who you are"...sorry, I warned you didn't I?
If you haven't explored Revit's Massing tools, take a look at the help documentation to get a sense of its purpose and capabilities.
Then try it out!!