All sanctioned tournaments in the Play! Pokémon program require players to submit a decklist when registering for the event.
From the 2014 Play! Pokemon TCG Rules and Formats:
Before the first round of a tournament, deck registration may occur. This process involves each player
listing the exact contents of his or her deck (and unused cards, in the case of Limited tournaments). To
make deck registration easier on the tournament staff, players should sort their decks to match their
decklists prior to registration. These decklists can later be used by Tournament Organizers and judges to
verify that a deck has not been altered since the outset of a tournament. Players are not allowed to
change their decks at any time during a tournament.
A decklist can be handwritten or typed on any piece of paper, but it’s easier to use the official Decklist or PokéGym’s unofficial decklist creator.