Invoice   Issued from the Seller to the Buyer, lists items sold and money owed to the Seller. The invoice is the 'base' financial document of any order/project. The invoice (also referred to as "Sales Invoice") represents the initial scope of that order/project and often is extracted from a Customer's Purchase Order.

Debit Note   similar to an Invoice (shows money owed to the Seller) but commonly used when goods or Services that are not listed in the initial Invoice are added to an order or project (for example Buyer wants to add 10 pieces of size L to the initial order, 2 pcs more then ordered have been produced, etc). Normally a Debit Note is not used as a 'standalone financial Document but comes 'in pair' with an Invoice.

Credit Note   opposite of a Debit Note (lists money owed from the Seller to the Buyer). Credit Note's value reduces the sum owed to the Seller. A Credit Note typically is used when i.e. 50 pcs of a product where ordered (an therefore invoiced) but only 49 pcs are being delivered. The Credit Note would list the item in question with a quantity of one. The initial invoice is left untouched (with the originally ordered 50 pieces)

The balance sum due for a project is calculated like this:

    Invoice total   +   Debit Note total   -   Credit Note total   -   payments processed   =   balance due

practically such a balance can also have a negative value (which means the Buyer has a "credit" with the Seller which either can be equalized by a payment or [in most cases] will be deducted from future payments to the Seller).