When Apple introduced Passbook in iOS 6 they billed it as a way to store all your tickets, coupons, boarding passes, loyalty programs, and other cards all in one place. It's hard to imagine they expected that place to one day be Microsoft Wallet, but thanks to Windows Phone 8.1 that appears to be exactly what's happening.

Passbook passes are just collection of data that get rendered into a card and displayed in Apple's Passbook app. Think of the Passbook files like the HTML, CSS, and JavaScript data that makes up a webpage, and the Passbook app like the browser that renders the actual page. It looks like Windows Phone 8 is taking the same data and rendering a similar card out of it, and then pushing that card to Microsoft Wallet.

Apple code-signs their Passbook cards, and it's not yet clear if Microsoft is simply accepting Apple certificates or accepting any pass regardless of whether it's signed or not.