A series is a group of three or more cards that share an appearance in artwork, name, or effect similarity, but are not genuine archetypes due to lacking any support cards. Series commonly comprise themed groups used by characters in the anime or manga.
From a practical standpoint, the most notable difference between a series and an archetype is the fact that many archetypes, such as "Lightsworn", "Blackwing" and "Six Samurai", can all have entire Decks easily built around them thanks to their own effects and the support cards they possess. Meanwhile, card series do not have support cards, and building a complete Deck out of a series such as "Hex-Sealed Fusion" can be a hard task. However, this can be also be an advantage, as some series-based Decks can be combined with other Deck types more easily than archetypes can, as they do not have specific support cards that could clog up Deck space. An example of such a Deck is Frognarchs.