Here you can enter the number of a particular item ledger entry that the line should be applied to. This means that the inventory decrease from this purchase credit memo line will be taken from the inventory increase in the item ledger entry that you select in this field. This creates a link so that the cost of the applied-to item ledger entry is carried over to this line. To see a list of the entries that you can apply to, click the AssistButton in the field.