Files with passbook extension might have been in the past encountered as some kind of tickets generated by long discontinued apps. Nowadays passbook is mostly associated with Apple Passbook, now know as Wallet.
PASSBOOK file extension - Passbook ticket
What is passbook file? How to open passbook files?
File type specification:
File extension passbook was in the past used in programs and apps to save various tickets and other similar documents.
More recently, passbook is de facto only known as an app from Apple's iOS system which is now known as Wallet.