What are top level objects?

What are top level objects in a开发者_如何学Gon iPhone application nib file?

About the Top-Level Objects

When your program loads a nib file, Cocoa recreates the entire graph of objects you created in Xcode. This object graph includes all of the windows, views, controls, cells, menus, and custom objects found in the nib file. The top-level objects are the subset of these objects that do not have a parent object. The top-level objects typically include only the windows, menu bars, and custom controller objects that you add to the nib file. (Objects such as File’s Owner, First Responder, and Application are placeholder objects and not considered top-level objects.)

https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/LoadingResources/CocoaNibs/CocoaNibs.html