An area on a document containing information to be extracted, where the information appears in a list of items.
For example, an invoice contains a list of items to be paid for. The list is divided into columns such as part number, quantity, and price. You can, for example, define three line item fields to use to extract the part number, quantity, and price for each item.
The above is an example of a "normal" line item field, which is found on purchase order invoices and can be used to associate purchased items with a purchase order in your ERP system. Functionally, bank account fields and tax tables are also line item fields.