Power units (tractors, box trucks, buses) are required to have 2 plates. The only exception is if they have a trip permit. Trailers do not need a US plate if they are for international freight.
So MX-plated trailer can start a trip in Mexico and end in US, or start in US and end in Mexico, but to ship domestically (start AND end in the US, it needs a US plate)
Also, MX-plated trailers can only be towed by MX-based carriers.