The container view is a view that you can drag into one of your view controllers that you already have in your storyboard (we'll call this viewControllerA). You automatically get a view controller connected to this view via an embed segue. This new view controller has its frame set, so that it's the same size as the container view -- if you resize the container view, the controller will automatically resize as well. So, if you want, you can drag in multiple container views into viewControllerA, and each will have its own view controller. In code, if you need to access these embedded view controllers, they can be accessed from viewControllerA.childViewControllers -- that will give you an array of any embedded view controllers that you have.
There is a discussion of these container views in the WWDC 2012 Session Videos video called "Adopting Storyboards in Your App".
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