OK, I tried something I saw on Envision CAD's Tips. I want to create a Corridor Boundary. The tip said to display the top mesh and use the Extract Mesh Boundary tool. But it complains that this would create a Circular Dependency.
The Element Information on the boundary graphic says it is a Linesting - there are no linkages or "Depends On" or anything else I can see that links this back to the terrain mesh.
It even retained the restriction after I tried using the clipboard to get a new copy.
However, I just discovered that a very small Max. Triangle Length behaves much differently than it did in InRoads. I set my length to 0.1 and the resulting terrain outline follows the extracted mesh boundary so closely that I have not found any location where they are different. If you set too small a value in InRoads, holes would begin opening up within the terrain. This now methodology is clearly preferred.
As I understand it, in ORD, corridors get an automatic terrain element, similar to how a Survey gets a terrain element, so this issue is probably not worth worrying about.
I hope others find this tidbit about Max. Triangle length helpful.