The Purchase Order Status options are pre-configured, but can be further customised.
When checked, automatically creates an invoice when this status is selected, and places it in Accounts – Invoices
Sync with Accounts
When checked, synchronises with your Accounting Software, if this has been previously configured.
When checked, it allows you to add more items to the order after it has been created. When you change the PO to a status that does not allow editing, then the Add more button will disappear.