Yellow A Alternate cards are reprints of specific cards from several Pokémon Trading Card Game expansions that feature a yellow "A" symbol next to their expansion symbol, indicating it is a print with alternate artwork.
Yellow A Alternate cards were first released in the Mega Powers Collection. This product included new Full Art prints of cards from the XY Series but the product itself was launched during the Sun & Moon Series. While reprints with different artwork were previously printed as Black Star Promos, in some cases, this allowed certain cards to stay legal in the Standard format longer than intended. To prevent this, the yellow A symbol was introduced. Yellow A Alternate cards have the same card number and expansion symbol of the original print with the exception of a letter added to the card number. As such, these alternate prints are only allowed in the same formats as the original print.
Two thirds into the Sun & Moon Series, many Yellow A Alternate cards consisted of Trainer cards with new artwork originating from Japanese subsets. A large number were also released in the Shiny Vault subset of the Hidden Fates expansion, despite featuring a different expansion symbol and number, as a number of these cards' original prints had already rotated out of the Standard format. Japanese expansions addressed potential card legality issues such as this by introducing regulation marks on cards from the beginning of the Sun & Moon Era. Subsequent prints of cards would retain their original regulation mark, ensuring their rotation regardless of any future prints. English cards eventually adopted this with the beginning of the Sword & Shield Series, eliminating the need for the yellow A system.