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 13.1 km² (5.1 mi²). 11.9 km² (4.6 mi²) of it is land and 1.2 km² (0.4 mi²) of it is water. The total area is ...