CMBS tracks an index that holds a basket of commercial mortgage-backed securities (CMBSs). These types of securities are backed by a pool of commercial mortgage loans on income-producing property such as shopping centers and hotels. The underlying index only holds investment-grade debt that are ERISA-eligible and have an expected life of at least one year. The fund uses a sampling method to track the index, meaning it will only hold a subset of securities with approximately the same risk and return characteristics as the index. Rebalancing is done monthly.