The software automatically converts the disc directly into a properly formatted WBFS file on your USB drive. Setting Up the Correct File Structure
Over time, the .wbfs format has become the for Wii homebrew users because it takes up less space than a full ISO file and works seamlessly with nearly all USB loaders.
This is the most user-friendly option:
Wii Fit Plus WBFS Link: The Ultimate Guide to Fitness on Nintendo Wii