A decrypted update file (whether official or modded) can be used in two primary ways:
On a Hacked 3DS (with CFW):
Whether you are on a flagship PC or an Android phone, the process is straightforward.
The search string specifies EUR USA... as two separate targets. While the Nintendo 3DS was technically region-locked, emulators are not—but the save files and DLC compatibility are.
Here is the golden rule: The region of the update must match the region of the base game.
The Crucial Difference for Emulation:
If you plan to use cheats (Action Replay codes via Citra) or mods that rely on memory addresses, the EUR and USA versions have different memory offsets. A "Walk Through Walls" code for USA 1.5 will likely crash the EUR 1.5 version.
Always verify your ROM's region ID (e.g., 0004000000055D00 for USA, 0004000000055E00 for EUR) before applying the update.