Generic Letter 94-04 was issued by the NRC to address a gap in the reporting requirements from the "old" 10 CFR Part 20 to the Revised Part 20.
Under the "old" Part 20, licensees only reported the occupational worker's radiation exposure information when the individual terminated employment at the licensed facility. The individual's entire radiation exposure history while monitored at that site was to be submitted to the NRC. Under the Revised Part 20, the licensees are required to report each monitored worker's exposure information for each monitoring year.
This causes a "gap" in the radiation exposure records for workers who continue to work at the same facility before and after the implementation of the Revised Part 20. Since the workers never terminated, they were not required to submit under the "Old" Part 20, and since the Revised Part 20 does not require the submission of prior records, these worker's exposure is not contained in the REIRS system.