It sounds like you're referring to Xdelta (specifically Xdelta3), which is a widely used binary diff and patching tool—often called an "online patcher" because it can generate and apply patches efficiently, even for large files, without needing both old and new versions fully in memory at once.
Use Cases:
Next time you find a cool fan translation for a classic RPG or a challenging Kaizo hack for Mario, don't be intimidated by the .xdelta extension. Fire up your browser, patch it, and enjoy the experience exactly as the creators intended—hassle-free. xdelta online patcher
Overview
xdelta3) or a GUI (like DeltaPatcher).xdelta3 -d -s original.iso patch.xdelta patched.iso.Traditional XDelta patchers come with friction: It sounds like you're referring to Xdelta (specifically