Expansion Packs are for MMOs Only, and only unlockable after sufficient research.
They are supposed to be created when an MMO is starting to make you have more expenses than income, so if your MMO game is starting to become unprofitable and you don't want to create a new one, an expansion pack can be uselful
Expansion Packs can be created if you have researched MMO Support and Expansion Packs. They increase the profit of the MMO without decreasing the expenditure/creating a new sales graph. Expansion packs are individually made, reviewed, and marketed but are still part of the original MMO. You cannot be requested to make an expansion pack by a publisher. *Note* Expansion packs do not act like sequels, you will not get bad reviews and low sales if you make an expansion pack without placing a different genre/topic game in between.