Aggregate Trailer Mass (ATM)
The combined weight of the trailer and its full load when it is not coupled to a tow vehicle.
Gross Trailer Mass (GTM)
The weight of the fully loaded trailer imposed on the trailer’s axle when it is coupled to the tow vehicle.
Note: GTM will always be less than ATM as some of the trailer weight is transferred to the tow vehicle when the trailer is coupled to it.