The Arcana cleric's Arcane Abjuration feature may affect a celestial, elemental, fey, or fiend.
A Tiefling has infernal heritage and an Aasimar has celestial heritage. Is this sufficient for them to be affected by Arcane Abjuration?
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According to the basic rules, player races (such as tiefling or aasimar) are assumed to be humanoid.
Every character belongs to a race, one of the many intelligent humanoid species in the D&D world.
Humanoid is a separate creature type than fiend or celestial. There are no creatures with multiple types. Therefore tieflings do not count as fiends, and aasimars do not count as celestials.
Some humanoids have subtypes, however "fiend" is not a valid subtype for humanoids. For example, a tiefling NPC would count as "humanoid (tiefling)".