A pass type identifier is the identity behind an Apple Wallet pass program: a reverse-domain string, such as pass.com.example.loyalty, registered in an Apple Developer account. Every PKPass declares one, and every pass must be signed with the certificate issued for it. A pass type identifier plus a serial number uniquely names one pass in the world.
The commercial question hiding in this technical detail is ownership. If the identifier is registered under the brand's own Apple Developer account, the passes belong to the brand: a future change of wallet provider can keep the same identifier, and members never notice. If the identifier belongs to the vendor, the passes are tied to that vendor, and leaving can mean asking every member to add a new pass.
The identifier also anchors everything that happens after issuance. Device registrations and push updates are scoped to it, so whoever controls the identifier and its web service controls whether passes can keep updating. This is why pass migration between providers hinges on the identifier and its certificates more than on any other asset.
When evaluating a wallet platform, ask two questions early: can the pass type identifier be registered under our own Apple Developer account, and who manages certificate renewals? The best answer is yes to the first, the platform to the second.