For each class to which you wish to add exposure, you must decide which characteristics you seek. If you want large-cap equity exposure, do you select a fund that mimics the S&P 500? Or one that is built to replicate the Dow Jones Industrial Average? Benchmarks such as these are just rules-based portfolios. Meet the rule and the security is in. Some of the benchmarks are better known than others, but that doesn’t mean they are a better choice.
There can only exist so many S&P 500 ETF’s. To be competitive and stand out, many ETF sponsors now offer funds that are built using their own rules, or factors. A factor may be driven by sales, earnings, leverage, company size, valuation, momentum, or almost anything you think will attract assets. Or you can decide to buy an ETF that focuses on specific industry sectors such as telecom, or financials, or technology. Does anything about this sound passive? Not a chance.