The first response is to share your disagreement with your supervisor during the performance planning dialogue or review meeting. It is beneficial at that time to share your perspective and any supporting data you can provide and to invite your supervisor to do the same. This process may help you and your supervisor to come to an agreement.
In the case where considerable effort has been made to reach an agreement without success, you may want to add a reply to the performance form if you feel that it is inaccurate or there are mitigating factors. You will still need to sign the document – signing indicates that it has been discussed, and does not indicate agreement.