From Bulbapedia, the community-driven Pokémon encyclopedia.
Unlike other games, solitary versions of the core series games contain less pre-release information because they are based on an already existing game(s). Pokémon Emerald is no exception to this as very little was changed in relation to Pokémon Ruby and Sapphire.
The Battle Frontier was changed slightly in various stages of development. However, these changes are minor and strictly relate almost entirely to building placements and static objects, such as fences.
In addition to the tracks present in Pokémon Ruby and Sapphire, which includes the upconverted music from Generation II, Pokémon Emerald contains the tracks from Pokémon FireRed and LeafGreen alongside its own. While a few of these themes are used, such as in Altering Cave, the vast majority go unused.
A location header named "SPECIAL AREA" was added after the empty Kanto and Sevii Islands headers. There is no actual information on what would have been included in this area, so it might have been intended merely to be a name for an event. This is probably the same case with the location header named "SPECIAL" in the Generation II games with index number 0. However, it is read as "CELADON DEPT." in Pokémon FireRed and LeafGreen.