Gameitall 
If you own Sonic The Hedgehog 4: Episode 1 and are looking forward to Episode 2, Sega wants to give you some extra content.
Similar to the “Lock On” technology behind Sonic & Knuckles for the Genesis (where you can load Sonic 3 into the game to add Knuckles into the game.) Players with Episode 1 will gain access into “Episode Metal,” which adds 4 new episodes and the ability to play Metal Sonic.
Of course a big difference is that you don’t have to connect cartridges.
