A unique identifier that identifies a single line item or multiple line item (Combined Payment) order.
For the GetOrders and GetOrderTransactions calls, the OrderID field can handle a hyphenated OrderLineItemID value. OrderLineItemID is a unique identifier for an eBay transaction and is based upon the concatenation of ItemID and TransactionID with a hyphen in between these two IDs.
See the Field Index to learn which calls use OrderID.