A dump of the physical retail cartridge. Many users prefer XCI as it can "bundle" the base game and the 2.1.0 update into a single file for easier management. How to Update Your Game
The latest version of Metroid Dread , Update 2.1.0, brings intense new ways to test your mastery of Samus Aran’s most fluid 2D adventure yet. Version 2.1.0: The Boss Rush Update
You must be running Atmosphere or a similar Custom Firmware.
The inclusion of both acronyms in the search string usually means the user is looking for a "Multi-Format" release, or the file can be converted/installed depending on the user's custom firmware (CFW) setup.