It is an order of upper-level item which, from the standpoint of an order, is the source from which the order is made. Among upper-level orders, the order at the uppermost level (the order of the highest-level item) is called a highest-level order, and an one-upper level order is called a parent order. Each order is given the highest-level order number apart from (its own) order number, which enables the relationships among orders.

Related term: Order Relationship