The noseweight of your tow bar is the unsupported load that your tow bar will be able to carry. This will be determined by the lower of (1) the noseweight for your car and (2) the noseweight for the tow bar that you fit to the vehicle. Your handbook should tell you the maximum noseweight for the car and you should be able to find the noseweight for the tow bar you are interested in. If these two measurements differ you must be guided by the lower value.
You must ensure that the weight of your bike rack rack, plus all the bikes, doesn’t exceed your noseweight. You must also be sure the weight of your bikes doesn’t exceed the maximum load for a particular bike carrier.