When you want to know which trailer you may tow with a category B driving licence, it is about the Maximum Permitted Total Weight of the trailer or caravan and the Maximum Permitted Total Weight of your car. Permitted total weight is stated on the car's and trailer's registration certificates.
The trailer, which a regular driving license (B) allows, must have a maximum permissible total weight of 750 kg. Your car must of course be approved to be able to pull this weight. If your car can tow a maximum coupling weight of 600 kg, the actual total weight of the trailer must not exceed 600 kg.
If the trailer has a permissible total weight of more than 750 kg, the combined permissible total weight of car and trailer must not exceed 3,500 kg.
During a check weighing of the lorry scale, both car and e.g. caravan be placed together on a scale, including driver, passengers and luggage.