In international law and international relations, a protocol is generally a treaty or international agreement that supplements a previous treaty or international agreement. A protocol can amend the previous treaty, or add additional provisions. Parties to the earlier agreement are not required to adopt the protocol. Sometimes this is made clearer by calling it <answer>an "optional protocol"</answer>, especially where many parties to the first agreement do not support the protocol.
What do we sometimes call an agreement that supplements a treaty especially when few parties to the treaty support the protocol?
an "optional protocol"