Purchase Order feature can be enabled with the toggle in Order & Payment section.
With the PO option enabled there are 2 ways you or your clients can use the PO number during the checkout process.
The first one is on the widget: the 5th step now contains a field where customers can add numbers to order.
There is also an option for customers to attach a PO file which by default is optional. After uploading and clicking on generate Invoice button, you can find a user PO file on your back-office side.
Secondly on the back-office side, you can find the “PO number” field which will store value from the customer in case it was specified or you can insert value after an order is placed.
When an order comes with a specified PO number (entered with either of the two ways) “Generate PO” button appears on the last online payment page for end-user. This button generates a special type of document called “Purchase Order #<NNN>” which differs from the invoice in having a “PO number” line under the customer details section.