More specific holdings of a general category of assets. A sub-asset class is a collection of assets that have common characteristics within both the asset class and the sub-asset class. The sub-asset class also has attributes that make it different than the parent group of assets. 

Diversification across asset classes in a portfolio balances its exposure to risks and reduces the volatility of the overall investment. Sub-asset classes can further identify and diversify the risks associated with the superclass. For example, stocks is an asset class, and large-capitalization stocks is a sub-asset class.