From Bulbapedia, the community-driven Pokémon encyclopedia.
The half deck is a trend among Pokémon TCG players, particularly in Japan. In half deck play, both players use a 30-card deck instead of a traditional 60-card one and lay out three Prize cards instead of six. The format caught on quickly, in part because there is very limited space in most Japanese TCG Leagues and also because the games take less time and will be able to hold a person's interest longer. There have been numerous official half decks released in Japan. The 2013 Summer Battle Tournament held from August to September in Japan features half decks as one of the options of play in their Urgent Missions tournament.