A series fund is a type of investment company that offers multiple series of shares, each with distinct investment objectives, policies, or strategies. Series funds are treated as a single fund for tax purposes, but they allow investors to choose the series that best fits their investment goals. A regular fund, on the other hand, is a single investment company with a single investment objective or strategy. Both series funds and regular funds are subject to the excise tax on undistributed income if they meet the applicable requirements.