The PIDummy object represents an object that couldn't be restored in a collection restored from a persistence string using the PIGlobalRestorer object. It acts as a placeholder so the index locations of other objects in the list aren't changed from what they were when the list was persisted.
It emulates all PISDK objects that support the IPIPersist interface. Almost all of the methods of those objects will return the psePIDUMMYERR error. If you refresh (using IRefresh) a collection that has a PIDummy object in it, it will attempt to restore the object it represents and place it in the collection.