Not Just for PO Codes
Even though this area references PO Codes, it can be used for other codes as well and you are able to change the prompt for this information. As an example, you may need to collect a Funding Code, or a Department Code, etc. Even though the admin will continue to reference PO code in the order detail and in the report writer, the site user/buyer can be prompted for the name of the code that is needed.
In the image below you can see in the Admin > Sites > Site > Settings > General Tab > Checkout Section, that you can set the prompt that the customer will see for the code. In this image you can also see that on a site level you can also choose to list the codes based on the the Billing location, set the requirement to either optional or required and also set an option to allow a custom code that has not been pre-loaded for them.
Knowledge Base articles related to Purchase Order Codes
What functionality is available for B2B Purchase order codes?
How do I set up Purchase Order Codes?