The index measures the performance of investment-grade (as determined by S&P Dow Jones Indices LLC), tax-exempt, U.S. municipal bonds, reducing callable bonds before 2028, and only including non-callable bonds maturing in 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.