Chains for anything on the trailer that's on wheels or tracks. Straps for a load that isn't on wheels or tracks and under a certain weight but I don't know what that certain weight is.
Personally, I've broken the law many times by tying down tractors with straps. I see custom combiners tying combines down with straps up over the wheels so they don't sratch the paint. I don't know how they get away with it. Jim