When an I-485 is accepted at the Service Center, requests for FD-258 FBI fingerprint checks, fingerprint fees, and G-325A consular background checks are made by USCIS. Responses to these requests come back to USCIS and must be matched up with the appropriate files held in staging. File Maintenance "file connects" these responses to the corresponding files.
In a recent meeting, Nebraska Service Center has clarified why, some times, in cases of pending I-485 applications, USCIS approves AOS interchanging the applications of the derivative and principal applicant: