title: Tracking in-app purchases & multiple devices (iOS)
body: <p>I want to implement in-app purchases to my app.  I will be using the built-in product model where the IAP will unlock features within the code.  My questions are:</p>
  
  <ul>
  <li><p>how do I track the a purchase on a device (as to unlock the feature)?  NSUserDefaults?  The guide vaguely states "application preferences".  They are not referring the Info.plist, as this can't be written once on the device.  I also (obviously) want the user to back this up.</p></li>
  <li><p>how do I enable the feature across mutiple devices on the same iTunes account?  Do they have to "purchase" the feature on the other devices, or is there another way?</p></li>
  </ul>
  
  <p>Thanks and regards,</p>
  
  <p>--John</p>