A subtype is a property of a card that describes allegiance to a smaller group with gameplay implications.

Previously, they were called "groups" by Riot, old cards (Beta 7 Pack Mentality) and supplementary information. However, the term has been phased out in favor of "subtypes", which has been the internal name since the beginning. Official wording, 9 newer cards and additional material use "subtype" now.

Subtypes can be official or un-official. Official types are noted on the cards and in the API, but there are some groups of thematically coherent cards that are referred to by their collective name, e.g. Crimson units by 3 Crimson Curator. In this case, an unofficial subtype is created to mark cards that belong together.