Empty weight, carrying capacity and maximum number of passengers?
The mass in running order is the weight of your camper including all factory options and tanks (fresh water + diesel + gas) filled for 90 %. Besides, the weight of a conductor is taking in count (always 75kg). You can find the information about this weight on your Certificate of Conformity (=COC).
The Maximum Authorised Mass (MAM) is the maximum legal weight you can carry with your chassis. If you cross the MAM of 3.500kg you need to be in the possession of a driving licence category C or C1.
The difference between the mass in running order and the MAM is called the load capacity. This carrying capacity isn't only used for the luggage but also for extra passengers, additional options (e.g. satellite, shed, ...) In fact you're free to choose the usage of the carrying capacity. If you have a carrying capacity of 400kg but you always travel with four people there won't be a lot storage space left to store additional luggage or accessories. If you travel with two, you have the possibility to store more luggage or additional accessories. In fact there is only one general rule, don't exceed your legal MAM of your camper.
The maximum number of passengers: the designer determine the maximum number of passenger taking in account the load capacity and the amount of legal seats with a seatbelt.
If you have a camper with a load capacity of 400kg and you're travelling with four people of 80kg, you have already used 320 kg of your load capacity. When your additional options weight another 50kg there is only left 30kg of extra luggage. If you travel with only two people you will get an extra load capacity of 160kg!
Tips and tricks:
to have more load capacity you can travel with half-fulled tanks. This give the opportunity to carry an extra 80-100 kg of load capacity. If you pass a road inspection the authorities will just verify your MAM and the amount of people which are legally allowed.