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From Wiki.zanox.com

Private individuals are not required to disclose their tax details. The legislation popularly called the "law to combat tax evasion" added a paragraph 1a to article 14 of the German Federal VAT Law, obliging the company supplying the product or service to specify the tax number allocated by the tax office in all invoices. Credit payments made to affiliates by zanox affiliate correspond, in the terms of the German VAT law, to affiliate invoices to zanox.

Thus as of 1 July 2002, this change in legislation requires that VAT-liable companies based in Germany must specify their tax number when commissions are to be paid exclusive of VAT. As a VAT-liable business, you are therefore obliged to add the relevant tax details in "Profile" > "Contact information".