Metro-based apps can articulate themselves one more way: simply as features that they “lend” to other apps. One of Apple’s publicly-demoed features of the upcoming update to iOS is the ability to share things via Twitter. It exists because Apple hardwired Twitter right into the OS. In Windows 8, that same feature is available to you because you happen to have installed a standalone Twitter client on this machine. The Twitter app’s developer added a few lines of code that allows Windows to offer those core Twitter features system-wide, wherever it’s appropriate, without exiting the original program.
(Agreed; “contracts” look like the best feature of Metro, and iOS is sorely lacking anything similar.)