Investment income doesn't just include regular amounts like interest, rent, dividends and trust distributions – it also includes capital gains (the profit a fund makes when it sells investments for more than their purchase price). Some pension funds pay no tax at all and might in fact receive a tax refund every year if their share investments include 'franking credits'.
To get this valuable tax treatment, many pension funds need a special calculation done by an actuary. The actuary works out what proportion of the fund was paying retirement phase pensions during the year. If that proportion is (say) 60%, then 60% of the fund's investment income is exempt from tax. You'll hear this referred to as the fund's 'actuarial percentage', and the document confirming it is called an "'actuarial certificate'.
Do all SMSFs with pensions need an actuarial certificate?
No – not always. If your SMSF is entirely in retirement phase (meaning all members are fully in pension mode with nothing remaining in accumulation accounts), the fund's assets are automatically treated as supporting those pensions and no actuarial certificate is required. These are sometimes called '100% pension funds'.
However, if your fund has a mix of members in accumulation and pension phase, or certain members have large superannuation balances across all their funds, an actuarial certificate will generally be required to calculate how much of the fund's income is exempt from tax. There are also some situations where getting a certificate may technically be optional but not worthwhile – for example, if the potential tax saving would be less than the cost of obtaining one.

