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Purpose of This Notice. This notice provides information about operations specification (OpSpec) D091 change in terminology and meaning from the previously used term "substantial maintenance" to the new term "Essential Maintenance," to specifically address Required Inspection Items (RII). Also, we removed the listing of maintenance providers from OpSpec D091 because Title 14 of the Code of Federal Regulations (14 CFR) part 121, § 121.369(a) requires the operator to maintain the listing of maintenance providers in its air carrier maintenance manual. In addition, this notice provides information about revised guidance related to these changes for Federal Aviation Administration (FAA) certificate-holding district office (CHDO) managers and principal airworthiness inspectors who have certification and surveillance responsibility for 14 CFR part 119 air carrier certificate holders conducting operations under part 121.