The deregistration certificate (Abmeldebescheinigung) confirms that you have deregistered your address in Germany. You get this document when you do your Abmeldung. If you do your Abmeldung by mail or by email, the Abmeldebescheinigung is sent to your new address.

This is not the same as the Anmeldebescheinigung. You get an Anmeldebescheinigung when you register your address.