Difference between UIManagedDocument saveToURL and UIManagedDocumentContext save

I am debugging a master data issue in my app (running on iOS 5) where changes to existing managed objects are not persisted across app sessions. If I use [UIManagedDocumentContext save]

, nothing happens and my changes are lost the next time I restart the application. However, if I use [UIManagedDocument saveToURL]

, the changes are saved. Apple docs are unclear as to which one is the correct way to keep updates. Can anyone explain the difference between the two and when to use each method?