codicil

An addition or supplement that explains, modifies, or revokes a will or part of one.

An addition or supplement modifying any official document, such as a treaty.

So insistent was this demand that the Wyandot actually received a codicil to the treaty […]

Those loose ends were tied up in a little-understood clarification of Brexit called the Northern Ireland protocol, ratified in January 2020. It looked like a mere codicil three years ago; now it looks like a serious diplomatic blunder that could threaten Britain’s territory and the region’s peace.

Any appendix or addition.

If Nick answered a question Wani listened to him and then gave a flat little codicil or correction.

To add a codicil (to).