The Purchase Order (PO) number is a unique number assigned to a purchase order. It represents a confirmation of the buyers' intent to seal the deal.
When creating a Sales deal for a certain buyer you can insert a PO number by opening a budget sidebar and clicking on the Details, Edit details section.
When opening the Edit detail section, the edit form will open, giving you the option to change the PO number:
After the sales deal is won (on how to manage a sales deal see here) and the budget is created, the PO number is automatically transferred to the newly created Budget.
Note that in the case of recurring budgets, the PO number will not automatically transfer to future budgets. Only the first one will have the PO number specified, while for others, you will need to manually update it.
Also, when creating an invoice from the budget the PO number automatically transfers to the Invoice creating form,
and to Invoice itself.
As is the case with recurring budgets, the PO number will transfer only to the first invoice in the recurring interval. For example, if the recurring budget is set to monthly intervals, the first monthly invoice will have the PO number transferred, but the second, and later invoices won't. You can add those PO numbers manually.