To set a blanket customer PO (sales order reference) for every time a customer orders from you, enter this blanket PO in the Cust ref prefix field under the customer tab in the company. The customer or sales rep will then get an error message if it is not in this format when they checkout or authorize the order.
To set a required format for the reference to be in, use the format field. E.g. if it has to have 3 letters and 3 numbers enter AAA111 or if it needs to always be 4 numbers enter 1111. The customer or sales rep will then get an error message if it is not in this format when they checkout or authorize the order.