As a business owner, you may get a cash discount from your suppliers. The discount you get there and perhaps take advantage of is important information for OkiOki. Because in the case of a cash discount, the amount paid will not match the invoice amount. However, if OkiOki knows how much cash discount you get then in the future she is going to be able to factor in that discount when automatically linking the transaction to the invoice.
You can mention this cash discount on your supplier's contact sheet. OkiOki will also ask you for it if she notices that you link a transaction to a document yourself where the amount difference is less than 5%.