Earlier this month, Microsoft confirmed that Windows 8's marketplace -- which is a native app in the new operating system -- would not offer games with a rating over PEGI 16, ESRB Mature ("M"). This rule, which resides in section 6.2 of the Windows App guidelines (opens in new tab), applies to all software sold within Microsoft's new virtual storefront and would mean no PEGI 18 or rated 'A' content in the store. The new system affected European users more because some PEGI 18 games are actually rated 'M' in the U.S.
Here in the States, games sporting an 'A' rating would be banned from the Windows 8 store under Microsoft's guidelines. However, because a number of extremely popular titles carry a PEGI 18 rating in Europe, they were covered by the ban even though they're rated 'Mature' in the United States. These include The Elder Scrolls V: Skyrim, Call of Duty: Modern Warfare 3, Mass Effect 2 and 3, Fallout: New Vegas and many more.
Today, Microsoft confirmed to Gizmodo that the company would now determine which titles are available through the store using a different system. In short, if a game carries an M rating in the United States but carries a PEGI 18 rating in Europe, it will still be allowed in the Windows 8 store.
The company's Windows App guidelines have yet to be updated, and still use the same language as before (nothing over PEGI 16 or ESRB M), but we're sure Redmond will get around to it.