An annuity account is an account earning interest into which you make periodic deposits or withdrawals.

A retirement account account is a great example of an annuity account. When you are working, you are making period deposits into the account; when you retire, you begin taking period withdrawals from the account.

We use Formula C when there is a one-time deposit (so we make the one-time deposit and let that amount grow over time).

We use Formula F when there are recurring deposits (so we are continuing to make deposits over time).