Relating to accession; pertaining to the act of acquiring, adding, or joining, especially regarding new items added to a collection.
From accession plus -al (making it an adjective). Accession comes from Latin accedere (to add to), and -al is a common suffix for creating adjectives.
Museums use 'accessional' all the time—when a painting enters a museum's collection, it goes through 'accessional procedures' with an 'accessional number,' creating a formal record of when and how the artifact was acquired!
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