If you've read the wonderful Revit series "Introduction to Revit Arch" & "Mastering Revit Arch" (both solid books, thanks to the authors, whom I've been meaning to blog about), you should already know that certain objects will show-up, even if they are above the cut plane of the view. All of the categories are categories that are "cut" in plan and it mostly has to do with architectural convention about showing some stuff in a plan, even if it is not "technically" visible in the plan.
So that works really great, for those categories. However, what if you have a light fixture (say a wall sconce) that you also want to show in your plan, but of course it is mounted at 5'-6" or 6'-0", what do you do! Even if you put in symbolic geometry it won't help, as the view is not cutting the object, or the object is not below the cut plane. One solution would be a plan region, but they can be wonky at best, and would you really want to do a plan region for every wall sconce...? Or meven worse, modify the cut plane for the whole view? More then one view
Enter the Invisible line, an unseen friend! Take your wall sconce family, and you'll note that among the various Line Types (really subcategories) available to you with which to draw lines is "
Now wait! You might say, what is going on. Quite simply Revit is obeying its own rules (kinda like Kirk in the Kobayashi Maru, but not really), while we can not "see" the invisible line, when Revit's view cuts through the line, its there, and it "sees" it, which means it sees the object. Since a light fixture is a "non-cuttable" category, that means Revit must render the fixture in projection, since this is a plan, that means to render the light fixture in projection would require that the "top" of the fixture be shown in the view. Therefore, the light fixture is displayed in plan the way you want it to, all because of a simple invisible line.
I've not done an exhaustive review, but I beleive invisible lines are only availble in families that belong to categories that do not cut (which would make sense), I'm also not sure if the line style shows up if you make a family first with a generic template, then change the category. I'll leave it up to you the reader to explore and learn on your own. I do know it works for light fixtures and speciality equipment (the two I deal with most often, and where the issue is most likely to crop up).