This method returns the persistence string for the object.
The object placeholder is an object that evaluates to a reference to an IPIPersist interface. See the Applies To list in the IPIPersist definition for a list of objects that support this interface.
PIModule's can have different values that are in effect at different times. When a PIModule is persisted, the EffectiveDate of the module is persisted too. When a module is restored, the value in effect at that date is returned.