When used with Bill Import, the transaction you are linking to via this field must be a PurchaseOrder: links to other transactions are not currently supported.
The LinkToTxnID aggregate allows you to link to an entire transaction, pulling in all of its line items. If you want to pull in only some of the lines from a linked transaction, you would not use this element, but would instead use the LinkToTxn aggregate within the line item. You can use the LinktoTxnID element along with the line item LintToTxn aggregate, but they cannot point to the same linked transaction, for example, they can’t both refer to the same purchase order or the same sales order. (That is, the TxnID in the LinkToTxnID element cannot match any TxnID in any of the LinkToTxn line item aggregates.) If you do this, you’ll get an error.
