It is an ad hoc item, usually compiled and executed after the main document, which contains additional terms, obligations or information.
In today's business world additional authorisation subjects such as company seals are not usually required unless stipulated in the original agreement.
These are called: an appendix (general term), an annex (which includes information, usually large texts or tables, which are independent stand-alone works which have been included in the contract, such as a tax table, or a large excerpt from a book), or an exhibit (often used in court cases), Similarly an attachment is used usually for e-mails, while an enclosure is used with a paper letter.
Outside of contract law, exhibits are often used in legal documents filed with a court as part of judicial proceedings such as motions, briefs and the submission of different types of evidence for inclusion in the record of trial of a particular case.
The addendum is the radial distance from the pitch circle of a cogwheel, worm wheel, etc., to the crests of the teeth or ridges.