But what does it all mean? Basically, an API is the publically available set of data that Blizzard makes available to addons and addon makers worldwide. The 3, in this instance, means "This account has Cataclysm, and you should adjust your behavior accordingly. 85 is max level." Conversely, 4 means "Hey, this account has Mists of Pandaria, and max level is 90" -- but many an addon hasn't been set up for this, and indeed, it's not possible to actually level to 90 right now.The API call GetAccountExpansionLevel() is returning 4 for accounts that have pre-ordered Mists of Pandaria on Battle.net, instead of 3. This is causing several addons to break, where they don't check for GetAccountExpansionLevel() being any higher than 3 (for level 85). 4 is supposed to be for level 90.
... Any addons that check to show a UI element or not for experience will now behave improperly.
If you haven't pre-ordered, the API call returns 3.