As Martyn stated, there are wide variations even within the same state. When we built our first one we were told pictures of all four sides and construction method. We went with "welded steel frame" for the chassis description. Came back as rejected...they wanted steel sizes, wall thickness, type of welding (stick, MIG, TIG...), welding rod or wire type... Maybe we caught a newby on a bad day with a new book to study!
Still, no matter what happens at this stage, nothing but the true regs matter when you are at the side of the road talking to your state's finest or the DOT. It is best if you know the regs as they apply to YOUR trailer. Here in PA you are required to have the trailer inspected every year, checking for DOT compliance and working systems. Seems that it is not done properly quite often, but it still becomes your responsibility in the event of a "incidence", DOT check, or if "the man in the unmarked Plymouth pulls you over to say Hi"!
Mitch