There should be no traces of loose mud, soil, gravel, lime, asphalt, or sand on, or inside, the cargo, etc. A common problem is “road dust”, which is a thin film of dirt that adheres to units during the road transportation.
“Dirt splashes” is another issue where dirt adheres to tracks, tires, panels, and cargo carriers, mainly during cargo operation. Accepted levels of road dust and dirt splashes are unfortunately hard to judge and a quality representative from the carrier should be consulted for advice if seen needed.