A complex certification too (as if any certification is simple). Show compliance for FAA certification of the 767C amended type design, for certification of the supplemental type design (installations of all the "tanker" items) and military certification of the many GFE bits and pieces for which there are no FAA criteria and which may be classified. And there's the Plan for certification and the actual Execution, which require many iterations to reconcile. These were all part of the USAF requirements and Boeing proposal the USAF selected.