During the VAT registration process in Germany first the seller received a DE Tax ID. This is a local tax registration number.
At a later stage the DE VAT number is issued that is valid for EU transactions. This VAT number always starts with DE.
The DE Tax Certificate is a document issued by the German Tax Authority proving the seller is compliant with the German tax rules and registered for VAT in Germany. Please note that the German Tax Certificate is necessary to start selling on Amazon.de