ok I can see how it would be confusing. Let me try and clear it up. code 15-3 is for the OPDS sensor being faulty. Early troubleshooting procedures had the technician testing the sensor and replacing the sensor, which is in the seat. Then if the code did not go away the next step was to replace the SRS control unit, which is the main control unit for the whole system.
If the dealer is telling you that then they are using old documents that should not be used.
The latest and most up to date document is a updated bulletin put out by Honda/Acura.
The bulletin states to replace the OPDS control unit, which is in the passenger seat, FIRST. further research by Honda indicated the problem was caused more by the control unit than the seat sensor.
If for some reason the code did not reset, then you would need to replace the seat sensor.
and on the very rare occasion that did not solve the issue, then the srs main control unit would have to be replaced.
Does that make it clear for you now?
The dealer has access to the same bulletins that I do, so I don't understand why they would be leading that way.