Using LAST in a map rule to reference an output
In a map rule, when referencing an output, LAST refers to the last built occurrence of that object. For example, using the same expression:
PO# Field:Order = PO# Field:Order[LAST]
LAST refers to the previous occurrence of Order because, when this rule is evaluated, the last known Order is the last one built.
LAST is a reserved word and is not case sensitive.