There are two ways you can show a purchase order number on an invoice.

Depending on the client's requirements they may have a purchase order number per shift or a purchase order number that is the same for all shifts in a specific timeframe. i.e. ongoing until advised otherwise. 

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You can instruct the system to show Booking Order Number on invoices.

To do this -

Refer picture "A" below.
If there is any reference code entered in the "Reference Code" field in the Client module on web then this code is displayed on the invoice.

If it is empty, nothing is displayed on invoice. This is just for your understanding that where is the reference code coming from.

To setup "Order No." visibility on the invoice -
Check / tick the option "Show shift order number in Invoice reference code" under the obligations tab for clients in the Finance module.

Picture "A"