Pokemon World

The Pokémon world that is currently available in PRO consists of 4 regions. Kanto, Johto, Hoenn, and Sinnoh. Aside from that there are also the smaller areas of the Sevii Islands, Love Island, and the Valley of Steel. The latter would technically be considered part of Hoenn.

Kanto
Kanto is the first region. This is where you will start your adventure.

Johto
Johto is the second region in the story. You need to beat the Kanto Elite 4 to enter. All pokemon caught in the Kanto region will be locked until you have gathered all 8 Johto badges.

Hoenn
Hoenn is the third region in the story. You need to beat the Johto Elite 4 and then beat Youngster Joey who will only accept your challenge after presenting him a Ratatta lvl80+ of which you are the original trainer. All pokemon caught in both Kanto and Johto will be locked again until you have gathered all 8 Hoenn badges.