Well, the folks over at Blizzard Entertainment have decided to bring this feature back to StarCraft 2. This basically means that any player can play the real-time strategy game for free.
All you need is the Starter Edition of StarCraft 2 and a friend who owns the game. By joining a party that includes a player with a higher expansion level, the players with lower expansion levels are immediately spawned up. In other words, players do not need to purchase StarCraft 2 in order to play some of the features that are exclusive to any of the expansion packs.
Naturally, this feature only applies to multiplayer parts of the game. Players of a party can not only play against the AI, but also play any Arcade game, custom game, and join the 2v2, 3v3, or 4v4 ladder together. If you still have questions relating to the new Spawning feature in StarCraft 2, check out the video below which explains all the important details.