Where are legendary pokemon "banned"?

Solution 1:

In areas such as the Battle Frontier, the game puts restrictions on the Pokémon you can bring in. The Pokémon banned from the Battle Frontier in Emerald are:

• Mewtwo
• Mew
• Lugia
• Ho-Oh
• Celebi
• Kyogre
• Groudon
• Rayquaza
• Jirachi
• Deoxys
• Egg

In the competitive battling scene, Pokémon are categorized into different tiers. The most used tier is the Smogon Tiers. The tiers are Uber, OU (overused), BL (borderline), UU (underused) and NU (neverused). Only NU Pokémon can be used in NU battles. NU and UU Pokémon can be used in UU battles, all the way up to Ubers where all Pokémon can be used.

Note that tier lists change from generation to generation as Pokémon gain new movesets or counters.

So to answer your question, Latios can be used in the Battle Frontier.