It is called addendum to the appendix or to the series of annotations that will be added to a written work once finished and that will have the mission of making clarifications, completing it, or failing that, rectifying any issue that is exposed in it.
It should be noted that this concept is used almost exclusively in written works, such as books, contractual documents, technical manuals, instructions, legal texts, medical treatises, among others. As with the appendix, the addendum usually appears arranged at the end of a book or any of the previously mentioned writings.
In the case of books, the addendum usually has the mission of accompanying the central work and explaining, in particular, the inconsistencies that emerged from it after it was published and that, since it was already finished, it is necessary to manifest them outside the main body and by this is that they are added to the end of the text. This usually occurs with those works that have already been printed and in which it would be really expensive to print them again, so the addendum resource is the best to reduce the cost and allow the reference of errors.
For their part, legal contracts also usually have addendums when it is necessary to change, expand or define some of the conditions set forth therein. Normally, the addendum will allow the addition of specific details or conditions that were not established in the original for whatever reason but that are essential in a contractual relationship to avoid future problems.
In manuals or instructions, the addendum is a very common resource since it allows adding certain information or a specific question that arises once it has been published.