When early Sega Mega Drive/Genesis emulators emerged in the late 1990s and early 2000s (such as Genecyst, KGen, and later Kega Fusion), they could not physically "lock" two files together. Emulators had to implement software-based lock-on features, or hackers had to manually stitch the two binary ROM files together.
It patches the Sonic 3 levels to allow Knuckles (with his gliding and climbing) to traverse stages that weren't originally designed for him. sonic-knuckles-wsonic3.bin