Load and Mass
The design of a trailer considers three main aspects of mass
- The load
- The mass on the trailer wheels
- Tow ball mass or tow ball downforce
The load is all of those things that you put into or on the trailer.
The weight on the trailer wheels is the vertical downforce transmitted to the ground via the trailer wheels.
Tow ball mass or downforce is the force placed on the tow-bar of the towing vehicle.
Tare Mass is the sum of B and C. i.e. Tow Ball Mass + Weight on Wheels when the trailer is empty.
Each trailer has a maximum Aggregate Trailer Mass (ATM) specified by the manufacturer, this is the RATED ATM and is the maximum that A+B+C can be when the trailer is loaded.
The Operational ATM is the actual ATM at any given time . This must always be below the rated ATM.
In a similar way this applies to both Tow Ball Mass Operational v Rated
And Gross Trailer Mass Operational v Rated
In other words, the manufacturer specifies the rating for each of these items and your should ensure you stay under the maximum specified.