This is an optional feature that you can use for accounting purposes. Once the Purchase Order number has been added to the job, it will be included in all future invoices the Freelancer creates for the job.
You can add a PO Number in the WorkRoom.
While you can enter a PO number, you also get the option to attach any relevant files for documentation purpose.
You can also edit the PO number once entered by clicking on the edit icon next to the PO number.
Additionally, you may also enter a PO number once the invoice is generated by going into the Payments tab and then into Invoices.