NSPlaceholders
A set of methods that an object can implement to register default placeholders to be displayed for a binding, when no other placeholder is specified.
Overview
Individual placeholder values can be specified for each of the marker objects (described in Selection Markers), as well as when the property is nil.
Placeholders are used when a property of an instance of the receiving class is accessed through a key value coding compliant method, and returns nil or a specialized marker.