This way is one of a large number of similarly tagged two-node ways (perhaps there are one hundred in number) each of which has the property that one node is PROVABLY in the drainage basin of the Rhine, while the other node is PROVABLY in the drainage basin of the Danube (Donau).  This means that each such way must be crossed at least once by the Rhine-Donau watershed.  This is to allow the (complementary) watershed data itself (something liike "Europa Wassershied" in German, though I've probably spelled that wrong) which currently has no error-bars associated with it -- and indeed is provably wrong in a few places based on the some of these links.