Here are some basics about Purchase Order Codes:
Site > Purchase Order Codes
You can create pre-arranged purchase order codes in this area. If you want, you can also assign codes to a specific location when you create them by selecting a location from the drop-down "Limit to this Location" menu. (This would only work if you're running the site in Locations or Both for the Address Book Mode.) Depending on settings you choose elsewhere (more on that below), the PO codes can show up in a drop-down menu when the customer checks out, so that they can select from a list of codes.
Site > Settings > "General Info" tab > "Checkout" section
You should check the "Allow customer purchase order code" if you want to enable the collection of PO codes at checkout. If you've defined codes as detailed above, the user will see a drop-down list at checkout of any codes that are available to them. If no pre-entered codes apply to them, or if they need to enter a code that's not in the list, they will be able to do that.
You should also select "Required" or "Optional" from the Purchase Order Requirement menu if you want the user to be able to use a PO code.
The Purchase Order Prompt box gives you the ability to change the wording that asks the customer to enter their PO code.
Site > Site Users > [User] > "General Information" tab > "Additional Information" section
In this area, you can override PO settings that you entered for the site in general. The two things that can be changed are the Purchase Order Requirement and the Default P.O. Number. In the Default P.O. Number menu, you can either select from a list of pre-defined codes or click in the box and enter one manually.