iShares by BlackRock is promoting The Core, exchange traded funds, their proprietary iShares, that when combined allow an investor to buy the entire domestic market, forming an efficient core holding for the overall portfolio.
The Core portfolio strategy combines three separate iShares: the S&P 500 Large Cap (IVV), S&P 400 Mid Cap (IJH) and S&P 600 Small Cap (IJR). The combination of these three components deliver the total S&P 1500, which can also be done by simply buying iShares S&P 1500 (ITOT).
Taking either approach will get you to the same place: the total US stock market, which includes all capitalization segments, small-cap, mid-cap and large-cap, and style segments, growth and value, representing a broad-based index composed of U.S. equities.
Further, portfolio cores can be assembled and tailored horizontally by cap-size (Large-, Mid- and Small-cap) or vertically by style (Value, Core, Growth).
Why buy three ETFs when I can just buy ITOT?
If your plan is to buy it and hold it as the equity core of your portfolio, and you don’t anticipate moving far off that allocation, then iShares ITOT is a good choice. It proves 100% coverage of the S&P 1500 without overlap or concentration.
However, if you want to replicate ITOT, but feel there will be times you want to tilt a little more heavily toward one of the three cap sizes, then the three piece ensemble may appeal to your propensity to over or underweight your positions.
Either approach allows you to assemble portfolios that will not expose you to overlap or concentration. For instance, while IVV, IJH and IJR are all S&P iShares, they each track securities unique to their underlying index. You will never see a company in IVV in the other two. No overlap. No concentration.
Both overlap and concentration can present higher risk to an investor’s portfolio. They often occur when investors assemble their portfolios from many component investments from different mutual fund families, and even within the same fund family. Its not at all uncommon for investors to buy two or three large growth funds, thinking they each bring something different to the table, only to discover that there may be up to 80% or more overlap in the holdings.
The Core strategy eliminates the risk of overlap and concentration.
What about Bonds?
iShares got you covered there, too. 
The core bond holdings can help you allocate and diversify your fixed income positions by maturity and duration.
The one stop shop is USIG which delivers the total bond market with maturities from short to long. Like the equity approach to using unique building blocks, three component holdings can help you target or overweight specific maturity bands within the USIG: short bonds 1-5 years (IGSB), intermediate bonds 5-10 years (IGIB) and long bonds 10 years and beyond (IGLB).
To recap, The Core equity approach delivers 1500 domestic index constituents (U.S. companies) that are diversified by CAP SIZE (Large-, Mid- and Small-capitalization) and STYLE (growth and value), and The Core fixed income approach delivers domestic U.S. bonds from short to long maturities.
The Core approach may be for investors who want to achieve diversification without overlap, appreciate simplicity more than complexity, want to keep investment costs to a minimum and assemble a portfolio that can provide broad exposure to U.S. markets.