The Residence Certificate aka T.M. 6 is a document that is used to document one's legal address. This is specifically required for things like obtaining or renewing a Drivers License.
In the past, it was possible to use an affidavit certified by one's consulate or embassy. However, that is both more expensive (a fee is charged, it was $50 USD at the US Consulate in 2018), and in the case of Americans, the US Consulate no longer provides this service.