Here at the National Support Center we get tons of questions about how AutoCAD based software works as well as questions about how the Revit based applications work. Most of the questions are coming from the direction of "this is broken how do I fix it or "I can't figure out how to do this". From time to time we get a question that touches both sides of that spectrum. I recently addressed an issue that did just that and felt it needed to be shared. We had a question come in where the user was trying to import his AutoCAD border into Revit. He was setting up his office's Revit standards and was migrating their CAD standards into Revit. When he imported the title block into Revit, the only thing that came in was two arbitrary points that encompassed a very small area. When I looked at his drawing I found nothing in the layout tab but when I switched over to the model tab, there was his border. So I simply moved the border to the layout tab and eureka, the issue was resolved. Something was bugging me though. Why did this fix it? Was it that Revit only looks at the layout tab? I started asking myself what process does Revit take when importing CAD drawings.
I found the answer when I took another look at the original drawing the client gave me. I kept feeling like maybe there was something in the layout tab. Although nothing was visible on the screen I drew a crossing selection window over the extents of the layout tab. I found two small objects. They were points on the defpoints layer. Knowing that the real border was in the model space tab I deleted them and saved the drawing. I then tried importing that original file into Revit. This time I got a totally different result. A message came up that read "Import detected no valid objects in the files paper space, do you want to import from the model space?" I clicked Yes and in came the border. So the moral of the story is this; when Revit imports a drawing from AutoCAD it first looks at the layout tab. If it finds objects then that is what is imported. If nothing is found then the user is prompted if they want to check the model space tab.