The index measures the performance of investment-grade commercial mortgage-backed securities ("CMBS"), which are classes of securities (known as "certificates") that represent interests in "pools" of commercial mortgages. The fund will invest at least 80% of its assets in the component securities of the underlying index, and it will invest at least 90% of its assets in fixed income securities of the types included in the underlying index that BFA believes will help the fund track the underlying index.