The HPDE Rules state:
11.4.12 Seats
All seats must be securely fastened to the structure of the car such that they are strong
enough to withstand a major impact. If replaced, the replacement seat should be installed according to the manufacturer’s instructions.
If stock seats are to be used with a roll bar/cage, care should be taken to prevent the seat from submarining under the rollbar. Care should also be taken to prevent the occupant from hitting his/her head on the roll bar/cage.
Passenger seats must meet the same requirements, per the CCR, as the driver seat, if used by passengers. Note- The passenger seat does not have to match the driver’s seat.
In the section on Belts for HPDE there is a reference to the racing rules but not in this section.