I suppose you're talking about the difference between compound interest and superannuation questions?
It's all about the question:
Compound Interest: Use the formula when the question specifies that there is an amount being deposited ONCE only initially and is being compounded by an % rate.
Superannuation:: The question will generally give it away, but if it involves the use of installments (person is depositing every time period), then you would use the A1+A2.... formula.