In projecting future revenue from the Buffett Rule, the Joint Committee on Taxation followed its standard practice of using a "current law" estimate, which assumes that temporary tax cuts or increases take effect as planned under current law.
Under the current law baseline, the so-called Bush tax cuts, which were extended in 2010, would expire at the end of this year. That scenario that would result in hundreds of billions in additional revenue from taxpayers, and the $47 billion would be on top of that.
A "current policy" estimate, on the other hand, refers to what the revenue numbers would look like if the current rules continued -- if Congress again extends the Bush tax cuts, as it is generally expected to do.
The current policy estimate says that if the Bush tax cuts are extended, then the Buffet rule would raise $160 billion over 10 years, or $16 billion a year. That's more than three times as much revenue as the current law estimate.