From Bulbapedia, the community-driven Pokémon encyclopedia.
Each location in the Generation III games has its own identification number used internally by the game, mostly to display where a Pokémon was caught. They are enumerated here, in order.
These identification numbers are only used in Generation III, and are not used to determine the location on the status screen of a Pokémon that is migrated to Generation IV—this is instead generated based on the game in which the Pokémon was created, being "Hoenn" for Pokémon Ruby, Sapphire, and Emerald and "Kanto" for Pokémon FireRed and LeafGreen.
Locations with index numbers between 000 to 087 appeared in Pokémon Ruby and Sapphire (and in most cases Pokémon Emerald), those with index numbers between 088 and 196 appeared in Pokémon FireRed and LeafGreen, and those with index numbers between 197 and 212 appeared in Pokémon Emerald. However, all games have the locations with index numbers 254 and 255, which correspond to in-game trades and fateful encounters; Pokémon obtained in these ways replace the formatting for met locations with their own special formatting (including no location word being highlighted).
In Pokémon Ruby and Sapphire, Pokémon whose met location corresponds to index numbers above 087 will display "obtained in a trade" rather than the location name. Likewise, in Pokémon FireRed and LeafGreen, Pokémon whose met location corresponds to index numbers below 088 or above 195 will display "Met in a trade" rather than the location name; however, in the trade center, unlike in Ruby and Sapphire, if the trade partner is the original Trainer of the Pokémon whose location does not exist in FireRed and LeafGreen, the game will display the met or hatched location as "Somewhere". In Pokémon Emerald, any Pokémon that was not transferred from Pokémon Colosseum or Pokémon XD will display the correct met location.
Locations shared between Pokémon Emerald and Pokémon FireRed and LeafGreen, such as Navel Rock and Altering Cave, have two separate index numbers, one for each region. Locations such as the Safari Zone and Victory Road also have two separate index numbers.