Some infractions in this document refer to “sets” of cards.
A Set is a physically distinct group of cards defined by a game rule or effect. It may correspond to a specific zone, or may represent part of a zone. A Set may consist of a single card.
A card or rule may create sets briefly during their resolution (such as Fact or Fiction, Collected Company, or even your draw for turn). If you are holding one or more cards, that’s a set. If you flipped over the top 3 cards of your library to reveal them, that’s a set. Other sets simply exist naturally in the game, such as your Library, Graveyard, Exile, and Hand. Just to be as clear as possible, it is also directly spelled out that a single card may be a set.
Cards are considered to be part of a set until they join another set. There is no in-between state and any card that has not yet been seen is part of the previous set (unless the new set is hidden).
This is just explicitly clear language to ensure that, no matter how hard you try, there will not exist a card without it being part of a set. Consider any unseen card as a part of the previuous non-hidden set.