Peter,
Here is a brief extract from Elsawin.
Description of function:

In order to empty the fuel tank as much as possible, a two-part reservoir housing with electric fuel pump is located in the tank.

The electric fuel pump delivers fuel from the outer part of the reservoir housing to a closed container (reservoir chamber) in the inner part of the reservoir housing. The injection pump of the diesel direct injection system draws fuel from this chamber.

In addition, the electric fuel pump forces fuel via a Y connection into a suction-jet pump below on the reservoir housing. The suction-jet pump ensures that the outer part of the reservoir housing is always filled with fuel.

In order to guarantee reliable engine operation, the chamber in the inner part of the reservoir housing must always be filled with fuel. If the fuel level threatens to sink due to an empty tank, the low fuel level sender -G210 gives a signal to shut off the injection system.