The index measures the performance of investment-grade (as determined by S&P Dow Jones Indices LLC), non-callable and callable U.S. municipal bonds expected to mature or be redeemed after December 31, 2027 and prior to December 2, 2028. The fund will invest at least 80% of its assets in the component securities of the index, and it will invest at least 90% of its assets in fixed income securities of the types included in the underlying index.