However, I'm unaware of any free insurance available with a USDOT #.
I seem to remember that from the post somewhere just before this thread was diverted onto USDOT.
Above though; we see a tractor - obviously in commerce. Possibly so, whenever there is no trailer hooked up to it? Could that be the key? Not for Hire.
So let's look at the Official Use Only designation:
Note that happens on all the license plate stickers.