Frontier markets can be thought of as countries that are even less financially developed and more difficult to access than emerging markets. As a result, these economies can offer even more long term upside than emerging markets with additional diversification benefits for many portfolios. Definitions differ, but MSCI's list of frontier markets includes names like Vietnam and Nigeria, for which there are standalone ETFs, but many other markets where no US-listed ETF provides direct access.
After this week, there will only be one US-listed frontier market ETFs providing diversified access across frontier markets: the iShares