In the past the FAA required applicants to certify each and every major repair that was done using a DER.  As time progressed the FAA allowed multiple S/Ns to be used on a single 8110-3 formwhich approved the engineering data used for those specific parts. They they allowed an engineering approval to exist as a dort of blanket approval. This proved a bit too lenient so a more formalized process was created using Major Repair DERs to approve multiple use repairs. The latest incarnation was to create a document called a Repair Specification which was a process agreed to by the applicant and DER and served as the multiple use approval. The latter stands today.