POP Series 9 was the first promotional set of the Pokémon Organized Play program to be released during the PL Series. Similarly to POP Series 6 and POP Series 7, the entire set is comprised of Pokémon. Unlike these previous sets however, all of the cards, bar 1 (Gible) were new.
While the set includes no Trainer cards, many of the cards featured have attack effects that rely on other Pokémon also in the set to be in play, making them somewhat useful in multi-type decks. This comes around from the fact that these cards were released as part of a "Collection Challenge" campaign in Japan, and were likely purposely made to complement each other.