A vehicle has several weight ratings which are based on it's various capabilites for braking, acceleration, and handling. These are assigned by the manufacturer mainly on the basis of liability. So while they are often underrated for a safety factor, it is not wise to exceed any given capacity, or all liability instantly reverts to the operator.
If any capacity of a vehicle is not given by the manufacturer, it is generally assumed to be zero, but some necessary data like the maximum gross weight rating must be suppied by regulation for all vehicles.
-Gross Vehicle Weight Rating (GVWR) is the maximum weight the vehicle can carry including the vehicle itself.
-Curb weight is the weight of the vehicle empty, unfueled, unladen, and without accessories.
-Gross Vehicle Weight (GVW) is the actual weight of the vehicle with all occupants, cargo, fuel, and accessories. This should never exceed the GVWR.
-Gross Combined Vehicle Weight Rating (GCVWR or GCWR) is the maximum weight the vehicle can carry and tow simultaneously.
-Towing capacity is the maximum weight the vehicle can tow, under any loaded condition.
-Tongue weight is the maximum downforce rating for a hitch displaced by the trailer tongue.
Many people forget that all these capacities are interrelated. Loading a vehicle to its maximum GVWR and then at the same time towing at maximum capacity overloads the vehicle. Actual towing capacity at any given time must reflect the weight the vehicle itself is carrying. If the vehicle is loaded to maximum capacity, the towing capacity will most likely be reduced.
As an example, a 1993 Chevrolet C3500HD truck is rated to tow 10,000 lbs, GVWR of 15,000 lbs, and GCVWR of 19,000 lbs. Fully loaded and fully towing at seperate capacities would place the entire combination at 25,000 lbs. So if the vehicle is towing 10,000 lbs, then the actual maximum allowable gross weight of the vehicle is 9,000 lbs. Since the net weight is roughly 8,000 lbs for this vehicle (not counting occupants, etc,) the carrying capacity of the truck's load bed in this case is almost nothing.
In addition to overall weight, load distribution must also be considered. Different parts of the system will have different maximum loads.
Excedeeding the tongue weight for a hitch is incredibly easy to do. Most common hitches are class III, less commonly class IV. A class III hitch has a tongue weight rating of only 600 lbs., while a class IV is still only about 1,200 lbs. Placing too much weight forward of the trailer axle(s) will force more downforce onto the hitch and less on the axle(s). Especially once the centerpoint is crossed between the forward trailer axle and the hitch.
On the other hand having too little weight on the hitch relative to the rear of the trailer will cause the trailer tongue to want to 'float' upwards. Ideally enough weight should be on the hitch to be under the tongue rating but still well above zero.
Each hitch will also have a maximum trailer weight, which may be different (especially in the case of a non-factory hitch) from the trailer weight rating of the vehicle. A vehicle might have a trailer weight rating of 10,000 lbs, but the hitch might only be rated for 6,000. In this case the hitch is the limiting factor.
It is important to note that individual parts of the trailer will have limitations as well, such as the coupler or the axles. These limitations should also be checked and never exceeded.
The load distribution of the trailer will affect handling and safety. The load should be balanced such that most of it is over the trailer axle(s), with enough downforce on the tongue to not exceed capacity of any one part but still allow enough weight to keep the hitch stable.
Too much tongue weight and the front of the vehicle will lift and impede steering. Too little tongue weight and the tongue may bounce and 'float' on the ball. Negative tongue weight, such as a case where the rear of a trailer is too heavily loaded, may actually lift the tongue and hitch, and the rear of the vehicle with it. Any of these unstable scenarios will decrease handling and saftey characteristics. Both of these situations can result in trailer sway which can be very dangerous
It is also important to note that a heavily loaded or imbalanced tow vehicle will also affect handling characteristics.
Some of the weight limitations of a vehicle may INCLUDE the use of trailer brakes. This may not be initially obvious and should be checked, especially if the towing capacity seems unrealistically high for a trailer without brakes. However this is usually overtaken by DOT (Department Of Transportation) mandates for brakes on trailers over a certain weight.