Does a vacuum truck usually use the pump to create vacuum in the tank, or air flow to suck up things?

I'm looking at vacuum trucks, and wondering about the function of the tank. the suction hose runs into the tank, then a few shutoffs and filters, to the pump. If the suction hose was blocked by a valve, the pump would create a vacuum in the tank. If said valve was open, the pump would just create airflow which would suck from the suction hose. Do you suck things up with the truck by creating a vacuum, therefore creating a pressure difference so the tank tries to balance the pressure by taking things up through the hose? Or do you just suck it up by running the pump and using it like a standard vacuum cleaner, where the tank is kind of like a vacuum bag?