The Fund is a fund of funds and seeks to track the investment results of the Morningstar® U.S. Bond Market Yield-Optimized IndexSM (the “Underlying Index”). The Underlying Index consists exclusively of underlying iShares exchange-traded funds (“ETFs”) (each, an “Underlying Fund” and collectively, the “Underlying Funds”), that themselves seek investment results corresponding to their own underlying indexes. The Fund invests primarily in the Underlying Funds, and cash and cash equivalents, including shares of money market funds advised by BFA or its affiliates (“BlackRock Cash Funds”). The Underlying Index is a broadly diversified fixed-income index that seeks to deliver current income while maintaining long-term capital appreciation. The Underlying Index is comprised of Underlying Funds within fixed-income sectors that collectively have demonstrated relatively high risk adjusted income on a consistent basis and meet liquidity characteristics as determined by Morningstar, Inc.’s (“Morningstar” or the “Index Provider”) proprietary index methodology. The goal of the Underlying Index is to represent an allocation to five distinct fixed-income sectors, with the exposure to each sector capped at a specified percentage of the Underlying Index. The five sectors currently included in the Underlying Index, and each sector’s respective current caps, are: between 10% to 50% U.S. government and government-related fixed-income securities, between 10% to 50% U.S. securitized fixed-income securities, between 10% to 50% U.S. investment-grade (as determined by Morningstar) credit securities, no more than 20% U.S. non-investment-grade credit securities and no more than 10% emerging market debt. These five sectors are represented by Underlying Funds that Morningstar, as the index provider, considers to deliver higher risk adjusted yields. Each fixed-income sector has its own return and risk profile. The combined allocation percentages of the five represented sectors will equal 100%. The Underlying Index is rebalanced and reconstituted quarterly. As of October 31, 2020, the Underlying Index included the following Underlying Funds within (i) the government related sector: iShares 1-3 Year Treasury Bond ETF, iShares 10-20 Year Treasury Bond ETF and iShares Agency Bond ETF; (ii) the securitized sector: iShares MBS ETF; (iii) the investment-grade credit sector: iShares 1-5 Year Investment Grade Corporate Bond ETF and iShares Broad USD High Yield Corporate Bond ETF; and (iv) the non-investment-grade sector: iShares iBoxx $ High Yield Corporate Bond ETF and iShares J.P. Morgan USD Emerging Markets Bond ETF. The Underlying Index may add, eliminate or replace Underlying Funds at reconstitution. The Underlying Index may include securities issued by sovereign entities. As of October 31, 2020, a significant portion of the Underlying Funds' Underlying Indexes is represented by corporate bonds and securities of companies in the financials industry or sector and mortgage-backed securities (“MBS”). The components of the Underlying Index are likely to change over time. BFA uses a “passive” or indexing approach to try to achieve the Fund’s investment objective. Unlike many investment companies, the Fund does not try to “beat” the index it tracks and does not seek temporary defensive positions when markets decline or appear overvalued. Indexing may eliminate the chance that the Fund will substantially outperform the Underlying Index but also may reduce some of the risks of active management, such as poor security selection. Indexing seeks to achieve lower costs and better after-tax performance by aiming to keep portfolio turnover low in comparison to actively managed investment companies. BFA uses a representative sampling indexing strategy to manage the Fund. “Representative sampling” is an indexing strategy that involves investing in a representative sample of securities that collectively has an investment profile similar to that of an applicable underlying index. The securities selected are expected to have, in the aggregate, investment characteristics (based on factors such as market value and industry weightings), fundamental characteristics (such as return variability and yield) and liquidity measures similar to those of an applicable underlying index. The Fund or each Underlying Fund may or may not hold all of the securities in the applicable Underlying Index. The Fund generally will invest at least 90% of its assets in the component securities of the Underlying Index and may invest up to 10% of its assets in certain futures, options and swap contracts, cash and cash equivalents, including shares of BlackRock Cash Funds, as well as in securities not included in the Underlying Index, but which BFA believes will help the Fund track the Underlying Index. From time to time when conditions warrant, however, the Fund may invest at least 80% of its assets in the component securities of the Underlying Index and may invest up to 20% of its assets in certain futures, options and swap contracts, cash and cash equivalents, including shares of BlackRock Cash Funds, as well as in securities not included in the Underlying Index, but which BFA believes will help the Fund track the Underlying Index. The Fund seeks to track the investment results of the Underlying Index before fees and expenses of the Fund. The Fund may lend securities representing up to one-third of the value of the Fund's total assets (including the value of any collateral received). The Underlying Index is sponsored by Morningstar, which is independent of the Fund and BFA. The Index Provider determines the composition and relative weightings of the securities in the Underlying Index and publishes information regarding the market value of the Underlying Index. Industry Concentration Policy. The Fund will concentrate its investments (i.e., hold 25% or more of its total assets) in a particular industry or group of industries to approximately the same extent that the Underlying Index is concentrated. For purposes of this limitation, securities of the U.S. government (including its agencies and instrumentalities) and repurchase agreements collateralized by U.S. government securities are not considered to be issued by members of any industry.