Crucially, you must have this update installed to access the game's key online features. Without it, you'll be locked out of:
Understanding Pokémon X Update 1.5: Decrypted 3DS CIA for EUR/USA Regions
EUR & USA Update Version: 1.5 (v51200) Format: Decrypted .CIA / .3DS System: Nintendo 3DS (CFW) / Citra Emulator
A "decrypted" update file has its AES encryption layers removed, typically via tools like 3DS-File-Decryption
Before you dive in and start changing everything, it's crucial to be aware of the risks.
General improvements to prevent game crashes during certain animations. How to Install (Citra/Decrypted)