Before work starts
Use an estimate when scope or quantities remain uncertain. Use a quote to set out a defined offer. A proforma describes a proposed transaction before the final invoice. Whatever the heading, clearly explain what is agreed and what still needs confirmation.
When placing and fulfilling an order
A purchase order comes from the buyer. A sales order records the seller’s understanding of the order. A work order gives instructions for carrying out a job. A packing slip lists what is in a shipment; it should not be mistaken for proof that the customer accepted delivery.
When money is due or received
An invoice requests payment. A receipt acknowledges money you have verified as received. A credit note records an adjustment to an earlier invoice; it does not by itself confirm a cash refund. A statement groups transactions so a customer can reconcile a balance.
Example: a small supply order
The seller quotes ten products. The buyer references that quote on a purchase order. The seller confirms the order, ships six units with a packing slip and invoices according to the agreed billing terms. The remaining four units stay visible in the order records, rather than disappearing from a revised document.