NTSC-U refers to the American release of Mario Kart Wii on April 27, 2008. It contains several notable differences to other versions including language support, bike and kart names, and also a partially different file system. The title ID is RMCE01.
To elaborate on the title ID:
- R - used for Wii games. Does not include WiiWare or Virtual Console titles.
- MC means Mario Cart, even though the series has been known with a "K".
- E is the region code for the US.
- The numbers 01 are used for the first-party games, meaning the games Nintendo published.