REDGUY: viz. dokumentace
From a practical perspective, in iOS and OS X outlets should be defined as declared properties. Outlets should generally be weak, except for those from File’s Owner to top-level objects in a nib file (or, in iOS, a storyboard scene) which should be strong. Outlets that you create will therefore typically be weak by default, because:
Outlets that you create to, for example, subviews of a view controller’s view or a window controller’s window, are arbitrary references between objects that do not imply ownership.
The strong outlets are frequently specified by framework classes (for example, UIViewController’s view outlet, or NSWindowController’s window outlet).
@property (weak) IBOutlet MyView *viewContainerSubview;
@property (strong) IBOutlet MyOtherClass *topLevelObject;
https://developer.apple.com/...ary/ios/#documentation/Cocoa/Conceptual/LoadingResources/CocoaNibs/CocoaNibs.html#//apple_ref/doc/uid/10000051i-CH4-SW6
TL;DR Top level je ten objekt, který je úplně nejvýš v celé hiearchii. např. v NIB souboru je to úplně ten první view, pod kterým jsou všechny. Většinou si ho jako strong bude držet controller v property view. viz. definice v UIViewController
@property(nonatomic,retain) UIView *view;