This call allows the seller to propose a partial refund solution (including the proposed refund amount) to the buyer. If the buyer accepts the partial refund proposal, then the next step in the flow is for the seller to actually issue the partial refund through the issuePartialRefund call.
This optional field allows the seller to add a supporting comment regarding the partial refund solution. For example, the seller might add a comment such as this in the field: "Since you are not 100 percent satisfied with the item, I will offer you a partial refund of $10 and you can keep the item." This comment is intended to be read by the buyer.