Sell
Invoice
Bill a client after delivery and get paid.
Quote / Estimate
Price the work before it starts; convert to an invoice later.
Proforma invoice
Confirm an order and terms before shipment or deposit.
Buy
Purchase order
Authorize a purchase and tell a supplier what to deliver.
Work order
Authorize a specific job, site and price before the crew starts.
Record the work
Delivery note
Show what was handed over, with a signature and no prices.
Timesheet
Log hourly work, total the hours, and pass them to an invoice.
Expense report
Group costs by category and total what should be reimbursed.
Correct
Collect
Not sure which one to send?
The short version: quote before, invoice after, credit note to correct, purchase order when you are the buyer. The guide walks through a full project sequence.