In an MS-Excel worksheet, cell B10 holds 90 and cell C10 holds 78, and their total in D10 is 168. The formula IF(B10 + C10 > 100, IF(D10 > 100, "A", "B"), "C") in cell D12 will result in

The outer test B10 plus C10 gives 168 which exceeds 100, and evaluation therefore moves into the inner IF.

The inner test checks D10 which is 168 and also exceeds 100, returning the letter A.

The letter B would appear only if the inner test failed, that is if D10 were 100 or less.

The letter C would appear only if the outer sum were 100 or less, which is not the case here.

The result AC is not possible because a single IF returns exactly one of its two outcomes.

Nested IF functions let one condition be evaluated only after another has already passed.