Sand Reset

[Hippowdon SaveSand]

The Sand Reset pokepower on Hippowdon lvx from RR states that I must shuffle all cards in play. What exactly is meant by "in play"? Does it affect the discard pile, because my friend and I have opposing views on it.

[PokePop]

The discard is not in play.
Neither is your deck, your hand, your prizes, or the Lost Zone.