The roof load that the vehicle manufacturer talks about in the manual is the dynamic roof load. This is the permitted load that you are allowed to carry on the roof of your vehicle while driving. For example, if you have specified a roof load of 80 kg for your vehicle, your roof rack + load may weigh a maximum of 80 kg.
The static roof load, on the other hand, indicates how much roof load is permissible when the vehicle is stationary. This is usually 3 to 4 times the dynamic roof load.