The Invoices screen displays a summary of all invoices for the account. An invoice is a type of transaction and has multiple line items that make up the transaction (e.g. taxes). You can dive deeper to see individual invoice details by clicking a specific invoice number. Within the Invoice Details screen, you can change an invoice's posting status or change a custom invoice status.
Invoices can also be generated immediately or in a "pending" status from the Invoices screen.