Formal:
* Addendum: A formal addition to a document, usually appended at the end.
* Annex: A separate document attached to a main document and forming part of it.
* Appendix: A supplementary section at the end of a document, providing extra information.
* Rider: A written addition to a contract or other legal document, typically attached separately.
* Amendment: A formal change or alteration made to a document.
* Supplement: An additional part that completes or expands on a document.
Informal:
* Add-on: A simple, informal term for an extra clause.
* Provision: A specific clause in a document that lays out a particular condition.
* Modification: A change or alteration, often used in a broader sense than amendment.
The best word to use will depend on the specific context and the formality of the document.