What is it and when does it come into play? I see DOT numbers on heavy duty trucks on down to pickups. Also, I've never used a section of interstate for deliveries where there are truck scales. If I did, would I have to stop if the scales are open? I'm driving a GMC 5500 with a gvw of 20,500 lbs. but it is plated for 24,000 lbs. I don't know off the top of my head what the individual axles front/back are rated for, but I would think that comes into play also. I may be driving through Canada with it in a month to New York state (not with firewood). Is there anything I should know before leaving, other than a passport?