walbut, based on a few public photographs, the modification appears to be similar to the existing 'Combi' or 'Quick Change' versions of the civilian 146.
The cargo door, and freighter roller-floor, enable customised pallets to be loaded. For Commercial operations the pallets would either have rows of seats, or a quick change to pure freight. Seats off-loaded remaining fixed to pallets; then freight pallets / bins loaded as required (there was a horse-box version). There was as I recall, one aircraft with a fixed configuration of mixed 'fish tank' and seats.
For medical, passenger cabin services probably use existing fold-down overhead 'baggage racks', but these could be converted for specialist equipment and fixed in place.
The pallet option would enable a range of medical / passenger configurations.
For certification, medical pallets might be treated as 'cargo', plus local approval for any special fit, e.g oxygen tent, but the seat pallets, services, and door, would be as the existing certificated 'QC' aircraft.