This is the invoicing screen. The invoicing screen is used as the workbench where you can create new customers, parts, services, accounts, jobs and other types of entities. From here you can also lookup invoices, customers, service history, products etc.
When you record an invoice, the stock is automatically updated. Recording an invoice also does your book keeping including but not limited to the GST reports, Profit and Loss, Next Service Reminder Letter, Customer Statements and many other standard business informational needs.
An invoice can be emailed, recorded and printed all at the same time from the Recalculate + Make Payment button.
There is also an Easy Invoice Screen shown below for inexperienced users that does exactly the same thing.