On the weights (VIN) plate there will be four weight limits laid out in the example plate I posted previously.
The top one is the Gross Vehicle Weight/Mass (GVW or GVM)
The second line is the Gross Train Weight/Mass (GTW or GTM)
These are both absolut legal Limits which must not be exceeded under any circumstances.
Maximum Towed Weight Limit (MTWL) does not appear on the data plate, because there is some latitude to the way it can be specified. Instead it is usually posted in the tow vehicles specifications. It will always observe the GTW limit for the vehicle, and whilst most car manufacturers opt to set the MTWL = GTW - GVW, which assumes the tow vehicle will be at its GVW limit.
Some manufacturers will calculate the MTWL based on a partially loaded tow vehicle, but the additional trailer weight allowed reduces as the load in the tow vehicle increases~ always observing the GTW legal limit. I must stress this method is not always used and is model specific.
Whilst the manufacturers MTWL is not a recognised legal limit, it must be considered as being one, if the tow vehicles MTWL is exceeded, and it is detected, it may be construed as an unsafe vehicle by virtue of exceeding a design limit and thus unroadworthy.