This is said to be an extremely rare device, that only a few Trainers in the Alola region own. It is a convenient tool that, as we all know, records information about the Pokémon that you see and catch on your adventure.
Professor Kukui will gift your character this special Pokédex to support you in your travels. It is inhabited by Rotom, a Pokémon that has the power to reside in various electronic appliances.
That means that it will do much more than recording Pokémon information, showing your current location, next destination, and advising what to do next based on your conversations.
Pokémon Sun and Pokémon Moon will release exclusively for Nintendo 3DS in North America, Australia and New Zealand on November 18th, and in Europe on November 23rd.