I think this is another step, and not as Steffen suggests a "cooking cabbages twice" phenomenon. I see it as perfectly reasonable that there should be a provision that documents passed to a third party should be subject to a confidentiality rule obligating that third party to respect the documents' confidentiality vis-à-vis their provider.

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Note added at 1 hr (2008-05-23 09:40:09 GMT)
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A note for Steffen: it is common practice in the UK to use the expression "Company confidential - for internal use only", or "Company confidential - not for publication", which each mean the same thing, the second part of the term being merely an expansion - or, if you like, a definition. The "Personal confidential" and "Third party confidential" are to be seen as two OTHER classifications - and different from each other. THAT is why I say your "nicht für Dritte" is WRONG. It means either "for internal use only" or "not for publication", but it does NOT mean "Third party confidential", as I - an English native speaker - understand the term.

If the English writer is suspected of having used the wrong term, then Michaela has to check this with the customer.