The "Sonic & Knuckles" cartridge features "lock-on" technology that allows it to be physically attached to the "Sonic 3" cartridge, creating a combined game, "Sonic 3 & Knuckles".
The "Sonic & Knuckles" cartridge features a "lock-on" adapter that allows it to be physically attached to other Genesis cartridges. Connecting the "Sonic 3" cartridge creates a combined game, "Sonic 3 & Knuckles".