This format is primarily used if you are modifying an original Wii U console. These files are transferred to an SD card and installed directly onto the console's internal storage or an external hard drive using homebrew tools like WUP Installer GX2. Playing Wii U Backups: The Two Main Methods

The Ultimate Guide to Wii U Emulation: Safety, Setup, and Preservation

Publicly uploaded ROMs are frequently incomplete, leading to game crashes mid-playthrough. How to Safely Build Your Wii U Library