The .smc extension stands for . In the early to mid-1990s, a company called Bung Enterprise created a floppy disk-based backup device called the "Super Magicom." This device allowed users to dump their original cartridges to disk. To identify its proprietary dumps, the device appended the .smc extension.
No. It does not change the game code. It merely allows emulators to read the file correctly. If the game ran poorly as SMC, it will run identically as SFC—or not at all if the header caused problems. smc to sfc converter