When a customer places an order, they get the option to fill in a PO number. This isn't a mandatory field though as some customers don't use PO's. Many customers refuse to pay an invoice without a PO though, so for those who do have this policy in place, I would like to be able to make adding a PO mandatory.
We are always going to have some customers who need a PO and some who don't, so a blanket rule for all isn't going to work. This would need to be something we could made mandatory only for certain accounts.