Have you ever collected sets of things? A new state quarter from every state in the U.S.? An autograph from every member of your graduating class?
In Mormon circles, those collectible sets might be a Book of Mormon in every language in which it has been published, or photographs of you shaking hands with each member of the Quorum of the Twelve in a given year.
Or a set of documents containing the signatures of every First Presidency throughout the span of the Restoration?