Pokemon TCG: Porygon-Z-GX Box - Rules of Play says:
Through a quirk of computer science, the Virtual Pokémon Porygon-Z has gained GX levels of power—though some say it is prone to overheating and requires careful maintenance! This Porygon-Z-GX Box features Porygon-Z in a rare public appearance, along with four Pokémon TCG booster packs and a beautiful oversize card that shows off Porygon-Z as never before!