The default value is this MonoTouch.UIKit.UIView.
Returns the MonoTouch.UIKit.UIView for which constraints relating to NSLayoutAttribute.Baseline should apply. The baseline for many MonoTouch.UIKit.UIViews is the bottom of the view but in a MonoTouch.UIKit.UILabel, for instance, the baseline should be the position of the text baseline.
The returned value must be either this or a view in this's subview hierarchy.