What's the difference between "physical damage" and "severe physical damage" in the NEC?
I don't know for certain but my interpretation of that language would be that "severe physical damage" would result from impacts or accidents caused by machinery, and "physical damage" would be an impact or accident involving people and objects only. For example, a forklift or pallet jack could easily crush even heavy-walled Rigid Metallic Conduit (the stuff with threaded ends and connectors). However if you had EMT conduit on the wall and a leaning 2x4 falls against it, it will probably be fine.