related part of the approval workflow to request change button

I am using approval action on a document set. I have already achieved this by the help of workflows: When the document is approved a choice option in the document set is changed to “A” and when it is rejected, it is changed to “B”.

I also need to change the choice option to “C” when the approver click on “request change” and submits the request change form. But I cannot find the related part of approval workflow for “request for change”.