Creating and configuring views is not all that you can do in Interface Builder. You can also connect the view objects in your XIB file to your application’s code. In particular, you can set target-action pairs and assign pointers.
In BNRDocument.xib, find the Placeholders heading in the document outline. A placeholder stands in for a particular object that cannot be specified until runtime.
One of these placeholders is File's Owner. File's Owner stands in for the object that will load the XIB file as its user interface. In your case, it represents the instance of BNRDocument.
Recall from the object diagram at the beginning of the chapter (Figure 32.2 ...