Nintendo gave the Super Game Boy peripheral a little upgrade in 1998: they added a transparent blue case, game link capability, and two LED lights that indicated when unit was turned on or when the game link was in use. The unit was released primarily in Japan, however some outlets in the USA did carry it (such as QVC, a home shopping network). The cost of the unit was around $60, and although its release wasn't hyped by Nintendo very much, the unit was available for sale.

The SGB2 functioned primarily like the original model. Gamers plugged the unit into the cartridge slot of the Super NES, then they plugged a Game Boy or Super Game Boy-enabled game pak into the SGB2. Turn the power on and the portable game is fully playable on TV. With the game link options gamers could link a Game Boy/Game Boy Pocket to the unit and play multiplayer games with other players. Of course, the other player must have a copy of the game pak being used in the SGB2. Presumably it's possible to link up two SGB2s together, although this would require a second Super NES unit and television. Furthermore, the game link port on the SGB2 could be connected to the Game Boy Printer and the Game Boy Camera. The unit cannot play dedicated Game Boy Color games, nor can it play Game Boy Advance games (seeing as how the SGB2 debuted years before the next-generation handheld gaming system did).


References:
http://www.gamersgraveyard.com/repository/snes/peripherals/supergameboy2.html