A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... the town has a total area of 23.9 km² (9.2 mi²). 21.9 km² (8.5 mi²) of it is land and 2.0 km² (0.8 mi²) of it is water. The total area is ...