The phrase "passenger seatback" refers to the backrest or support of the seat in a vehicle that is meant for the person sitting beside the driver. Full definition
Performance Design Heated Front Bucket Seats - inc: height adjustable driver's seat height and tilt adjustable head restraints front passenger seatback pocket and whiplash protection (chicagomotorcars.com)
Long items can also be accommodated when the front passenger seatback folds flat. (cars.com)
60 / 40 - split fold - flat rear seat and fold - down front passenger seatback for storing and transporting items up to eight feet long (gmauthority.com)