A quotation tells a customer exactly what they will get and what it will cost, before any work starts. Fill it in on the left and the page updates as you type.
A quotation is an offer at a fixed price. It is sent before the sale and does not ask for payment. An invoice is sent after the sale and does.
When the customer accepts, use “Make an invoice” in the editor. The customer and items are copied across, so the invoice matches the quote.