From Bulbapedia, the community-driven Pokémon encyclopedia.
A Department Store is the largest shop in a region. It can refer to:
Game mechanic limitation with rooftops
Even though most department store rooftops are open air (the stores in Generation IV are not), and seemingly a perfect place to fly from, a player cannot use a Pokémon's Fly ability to leave the rooftop. The player's Pokémon will only fly from the ground because the rooftop has been programmed as an "indoor" location.