From GNUstepWiki

XIB fields

NSCell is an abstract superclass, these fields are usually serialized by its subclasses.

NSControlView reference The Control that owns this cell, usually it's parent.
NSSound NSCustomResource (optional)The sound a cell should play when activated.
NSPeriodicDelay int (optional)The amount of time in ms before the cell should process incoming events after it begins processing one.
NSPeriodicInterval int (optional)The ammount of time the cell should appear to aknowledge an event(e.g. button displays as pressed)