PrF UG forms contains
ContainsWidget is an optional widget for slots of type Instance. The default widget, InstanceFieldWidget or InstanceListWidget, lists Instance(s) that are selected as the value of the slot. ContainsWidget actually contains a version of the form for each instance that is a value of the slot.
ContainsWidget is useful primarily for Class or Instance slots where the Class Editor for the Allowed Classes is fairly simple, and where the cardinality is single or small. For a widget that provides a summary of selected fields, see InstanceRowWidget or InstanceTableWidget.
For information on how to customize the display of a ContainsWidget, see Customizing a ContainsWidget.