It is a molded plastic pet carrier. It is a specially shaped and fitted container designed to provide comfort, storage, portability and protection to pets of various sizes. The top of the pet carrier has a foam covered plastic handle and a small circular door. The front of the carrier has a molded plastic cage door. Inside the carrier there is a removable plastic mat and water/food tray for the pet. When placed inside a car, the pet carrier snaps into a molded plastic base that is secured to the car seat by means of the seat belt.
[1] Please importantly note that only the relevant portion of the related is displayed above. You may refer to the said customs ruling for the entire ruling explanation.
[1]. https://rulings.cbp.gov/ruling/N010017