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.
My wife and I had our passports stamped after a 6 hour ordeal that started at 8:30 a.m. and ended at 2:30 p.m. Here are some tips based on what we experienced,
1. For an 8:40 appointment, get there before 8 if at all possible. They were not checking appointment timings, so people with 9/10 a.m. appointments were ahead of us in the line. All they care is that you have an appointment for that day.
If anyone's interested: I had an appointment today for H1B visa revalidation. Overall it was a smooth experience. I was asked for my paystubs though.
Degree etc was not asked for. Total interview time was 2 minutes. The only question I was asked was whether I am working for the same company for which I had received an H1 visa previously. In my case that holds true. I also had an F1 visa previously. US Degree.