The navigation DU is the inboard DU, appropriately situated next to the PFD. The ND can display a number of things, such as TCAS target reference, vertical and horizontal deviation for ILS signals, deviation bars for radio navigation, etc. The view in the screen shot below is the typical configuration for the DVA A330. In ARC mode you will see the frequencies your navigation radios are tuned to and where the aircraft is positioned in reference to these radio facilities. See below for different ND modes.