Monster Hunter - A New Age Expansion Packs

Monster Hunter 3 - A New Age has many Expansion Packs planned for release in the future. These expansion packs are optional, usually free, and they expand upon the game's already-included content. Some purchased expansion packs may grant access to special areas that can't be accessed any other way, while other ones may simply add to the game by installing more or better textures, a new monster, or something similar.

In Detail
Due to the nature of the client, only those monsters which are installed in your client can be seen, fought, or researched. This means that even if you find a quest hunting a monster you'd really like to see but don't have installed, you won't be able to fight it. Weapons, armor, and even certain character model changes work the exact same way.