From Bulbapedia, the community-driven Pokémon encyclopedia.
Each location in the Generation V games has its own identification number used internally by the game. They are enumerated here, in order.
Locations with index numbers up to 00116 appeared in Pokémon Black and White; those with index numbers between 00117 and 00153, as well as the individual index number 30015, are exclusive to Pokémon Black 2 and White 2. Index numbers between 00076 and 00105 are used for locations in another player's world when using the Entralink. Index numbers between 00154 and 30000, 30016 and 40000, 40110 and 60000, and above 60003 are filled with null data. If a Pokémon references an index number for met location that does not exist in that game, it will be displayed in that game as Faraway place.
Pokémon Black 2 and White 2 have all location headers programmed in that Pokémon Black and White do; however, the location ID for Cold Storage was repurposed as PWT, and likewise the Treasure Hunter became PKMN Breeder. Trading a Pokémon with one of these met locations to the other pair of games will cause its location to appear as the other value. Additionally, in Black 2 and White 2, locations that have physical maps tied to them also have an associated value between 1 and 5. In Easy Mode, this value will be subtracted from the level of each Pokémon that opposing Trainers use within that map, while in Challenge Mode, the same amount will be added to their levels.
Though there are pointers for "Kanto", "Johto", "Hoenn", "Sinnoh" and "Distant Land", these are not used for Pokémon caught in these respective regions. Instead, Pokémon sent via Poké Transfer all use the same met location, with the region displayed being dependent on the game of origin. However, "Hoenn" is used on the Ruby and Sapphire Anniversary Kyogre and Groudon.