When the insert or add item action is available and a transaction is identified for that purpose, the Set as contextual default for property identifies an attribute, in the preceding transaction. The attribute must have the Contextual default property set to true in the transaction.
The meaning of this property is: take the value of the field having this property and set it as the default for the referenced attribute, in the selected transaction. Usually, the "fields" having this property are:
See Contextual defaults for further details