A signed letter from the owner of the property authorizing you to register at the address. Used when you are not the contract holder (e.g., subletting a room).
Ask the owner to write and sign a letter stating they authorize you to be empadronado at the address. Include the owner's name, ID/NIE, the property address, your name and passport number, and the owner's signature. Attach a photocopy of the owner's ID.