Many investors use mutual funds for their brokerage accounts and the retirement accounts like traditional 401k, Roth 401k, Individual Retirement Accounts (IRA), Health Savings Accounts (HSA), and other long-term accounts.
Mutual funds are open-ended funds, which means they can issue unlimited new shares with the net asset value reestablished after the market closes. Closed-end funds offer a fixed number of shares, and no more are created.