The average cost accounting method was first created to allow a taxpayer to simply report the gain on partial sales based on the average cost of all shares purchased (instead of the default FIFO treatment, or by using specific share identification), but was reserved exclusively for mutual funds and not for individual equity securities.
However, it appears now that the rules may be a little broader than anyone realized - because technically, an exchange-traded fund (ETF) may also be eligible, notwithstanding the fact that it trades more like a stock than a mutual fund.