Two codes, different situations
PETC usually identifies a pet accepted in the cabin, while AVIH usually refers to an animal travelling in the pressurised hold of the same flight. Each airline applies its own limits.
What determines the option
Size and weight with the carrier, aircraft type, airline policy, route and connections can change what is available.
Before booking
Confirm acceptance of the pet in writing for every sector and check that the reservation contains the correct code.