Unfortunately it's not a clearly written clause. It's derived from the dignity of humans and similar general terms. So the interpretation has changed over the decades. As long as the GDR existed it was clearly considered a property of a totalitarian state. Existed in the GDR (East) but unthinkable in the Federal Republic (West). Maybe 15 years after reunification (don't quote me on the exact number) a permament nationwide tax id was introduced. With the promise that its usage will forever be limited to tax offices and exchange with any other authorities is illegal. Another 15 years later that promise was "forgotten" and now the tax id can be exchanged with many authorities. It's relatively new and probably not widely used yet. But the dams have been opened.