A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... town has a total area of 5.9 km² (2.3 mi²). 5.4 km² (2.1 mi²) of it is land and 0.6 km² (0.2 mi²) of it is water. The total area is 9.61% ...