Microsoft has revised its Xbox 360 Achievements rule book, agreeing to allow publishers to add an additional 250 Gamerscore points to disc-based games through add-on content. The content could be either free or paid. Xbox Live Arcade games can also receive a boost, as publishers are now allowed to add 50 additional points (to the required 200) via downloadable content, which could be similarly free or paid.
Each disc-based game must include 1,000 points that are accessible for free (excluding price paid to obtain game), but not necessarily included on the game disc; Crackdown, for example, will only ship with 900 unlockable points, while the remaining 100 will arrive built into a free content update. Microsoft admits that there have been "inconsistencies" in the past, but hopes that the new, supposedly clear policy will regulate publisher's Achievements.
Official rules (via Gamerscore Blog):
1. All regular disc-based games MUST have 1,000 Gamerscore in the base game. This means that any consumer who buys a retail game will have the opportunity to unlock the full 1,000 Gamerscore without having to pay for any add-on content. Note that a publisher may decide to deliver a portion of this Gamerscore via add-on content, but the add-on content will always be free to the consumer.
2. Game publishers will have the option to deliver another 250 incremental Gamerscore on top of the 1,000 via add-on content from Marketplace. This add-on content could be either free or paid. So if you complete a game and earn the full 1,000 GS, you could by getting new add-on content earn up to a total of 1,250 points from a game.
3. Xbox Live Arcade games will operate in similar fashion, but given the size of these titles they will allow you to earn up to 200 Gamerscore from every game and up to another 50 points from add-on content.