Passes are a digital representation of information that might otherwise be printed on small pieces of paper or plastic. They let users take an action in the physical world, in the same way as boarding passes, membership cards, and coupons. The pass library contains the user’s passes. Users view and manage their passes using the Passbook app.
Apple Developer PassKit Programming Guide
Your source for free iOS 6 Passbook passes; create your own iOS 6 Passbook pass!
Content-Disposition: attachment; filename="pass.pkpass"