When to use the Back button vs. Cancel/Done on iPhone

I noticed that in some cases Apple uses Cancel / Done buttons, and in others there's only the the Back button.

In Calendar, for example, when you choose how often an event should repeat, you have Cancel and Done, which automatically takes you back to the previous screen. However, in Settings, when you choose a ringtone, you have to hit Sounds (or Back) to save your choice.

My guess is if you're creating something, you have to hit Done at every step, but when you're just changing settings, the "Save" (or the Done button) is implied when you hit back.

Are there any official conventions for how and when to use these?