What is Apple Guideline on when to use modal vs push controller?

Apple's guide on when to use modal vs push-view controller when clicking on a table view cell seems confusing.

Take Settings-> Twitter for example. If you click Add Account Cell, a view will be entered to sign in. If you click on an existing account, a modal view will be shown instead. Most other places in settings use a pushed view instead. In Contacts edit mode. modal is used I can't find a general pattern on when to use.

What is Apple Guideline on when to use modal vs pressed view controller when clicking on a table view cell?