These types of files are used to experience the "definitive" version of the game, combining the best aspects of both localization and original audio, playable on alternative platforms.
A .rar file is a compressed archive containing the .3ds or .cia game file. EOU-TMG-(UNDUB)-DecrTD-Ziperto.rar
Here is an informative breakdown of what this file represents: These types of files are used to experience
If you're asking about this for technical or archival reasons, I can also explain: How to What the differences are between this and the original Where to find other similar modified ROMs This is a role-playing game developed by Atlus,
Etrian Odyssey Untold: The Millennium Girl (EOU-TMG). This is a role-playing game developed by Atlus, combining dungeon crawling with a structured story mode.
The file has been "decrypted," meaning the Nintendo 3DS encryption has been removed. This allows the game to be played on PC or Android emulators like Citra , rather than requiring a modified ("hacked") physical 3DS console.
Based on the file naming convention, refers to a specific, modified ROM file for the Nintendo 3DS game Etrian Odyssey Untold: The Millennium Girl .