Each aircraft, to be able to fly in most countries of the world need to have a Certificate of Airworthiness.
If the owner/operator of the aircraft want to modify (or repair outside the approved manual) the aircraft with additional installations, these installation need to be approved by the Authority in order for the Certificate of Airworthiness to remain valid.
A DOA is an EASA approved organization that can approve modifications (if minor) on behalf of the Authority and is the only organization that can work with EASA for the approval of a major modification (STC) or repair.
