You can customize forms such as Purchase Orders, Sales Orders, Invoice, Quote and Packing List if the default forms do not suite your needs. You can see a list of Custom forms under Enterprise Setup, Custom forms.
We provide a list of forms for invoices: Default Invoice, Invoice with 3 Party Header, No 10 Window Envelope Header. If you want to make Invoice with 3 Party Header or No 10 Window Envelope Header the default invoice form to use, you would click on the button with a checkmark on it. This will make the form the default for all new invoices created.