This is not about arbitration, nor is it about patents, but rather an objection against a decision made by the fiscal authorities. I would thus refer to the party raising the objection (against a tax assessment, for example) as the "petitioner." Some even refer to this objection as an "appeal," for which "petitioner" also work in addition to "appellant."

If the fiscal do not remedy the situation, the petitioner may file an appeal with the tax court (Finanzgericht), in which case the petitioner could then be named "appellant", or simply "petitioner" (see above).

See http://72.14.221.104/search?q=cache:mMulMc0gTYYJ:europa.eu.i...

Also see http://www.google.com/search?hl=de&rls=GGLD,GGLD:2004-29,GGL...
:-)

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Note added at 1 hr (2006-08-02 16:32:49 GMT)
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Sorry about the typos and omitted words, e.g. "...WOULD also work..." and "If the fiscal AUTHORITIES..." - I was typing to quickly.
;-)

Oh, and here's some more info: http://www.taxation.de/highlights.pdf