accessional
adj. C2 Proficiency ac·ces·sion·al
adj. relating to something that is added or extra. It is an old word that describes things that come after the main part of something.
adj. relating to or serving as an addition or supplement.
The library added several accessional books to the collection.
The legal team reviewed the accessional documents to ensure all supplementary evidence was included in the final filing.
While the primary treaty was signed in 1815, the accessional agreements that followed provided the necessary framework for resolving territorial disputes in the subsequent decades.
From accession + -al.