When creating a RTS purchase order - an amazing feature by the way :) - we can add notes to the PO. However these notes are not visible by the customer. Our customers are eCommerce sellers and have returns of items that consumers often leave notes in for them.
In an ideal situation, we would add this note to the comments field, then the confirmation email back to our customer would contain that note, or at least it would be visible on the PO from the customer's access.