4-4 window, Invoice Defaults there's an option to enable PO warning. Add a PO Warning for Receivable Clients in 3-6. This would be used for clients that have special billing requirements. For example, whenever you generate an AR invoice associated with a client, it warns you that you need to enter a PO (if there's none in the PO# field in 3-2). Another example would be, when you link a job to a client, it prompts you that the client requires PO.

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