If you’ve ever ventured into the world of Pokémon ROM hacks, you’ve likely seen a very specific instruction: "Use the 1635 - Pokémon Fire Red (U)(Squirrels) ROM."

Every Pokémon from Gen 1–9, Mega Evolution, Z-Moves, and extreme difficulty. Pokémon Gaia

It sounds like you’re looking for a guide or clarification on a specific ROM named for the Game Boy Advance.