The refundable alternative minimum tax (AMT) credit was a special provision in effect from the 2007 tax year through the 2012 tax year. It let you accelerate the use of AMT credits more than three years old and therefore helped some taxpayers recover taxes paid because of an ISO exercise that triggered the AMT.
The provision for refundable AMT credits ended after the 2012 tax year. Only the standard AMT credit provision is currently available. For details, see the FAQ on large unused AMT credits.