In both cases, once the invoice is created, it can be printed as a test but will actually only be recorded in incomes or expenses once published. It is possible to associate taxes
to a invoice so you can track them later through your tax accounts and through reports
Before you can use customer invoices, you must create at least on account
of type "Receivable", and in the case of vendors, you need at least one account of type "Payable".
Be careful, customer, vendor and invoice creation are not reversibles, so its better to practise yourself on a test file to learn how it works if you don't want to have many empty customers in your customer list.