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.
Here are some experiences with regards to my entire track from NIW I-140 petition to I-485 approval. I had somewhat of an unique case, so if anyone here is in the same boat, this may be of help to you. I have done the entire process by myself, without an attorney, and long hours of research in the law library on U.S.C. Section 8.
Arrived to US in 1991 on J-1, not subject to section 212e, changed to F-1 on arrival. Became an H-1 in 2001 and filed for NIW in March of 2002, case was backlogged due to 9-11.