Explanation
This formula is evaluated for each of the 10 cells in A1:D10. A1 will change to the address of the cell being evaluated, while C1:C10 is entered as an absolute address, so it won’t change at all.
The key to this formula is the =0 at the end, which “flips” the logic of the formula. For each value in A1:A10, COUNTIF returns the number of times the value appears in C1:C10. As long as the value appears at least once in C1:C10, COUNTIF will return a non-zero number and the formula will return FALSE.
But when a value is not found in C1:C10, the COUNTIF returns zero and, since 0 = 0, the formula will return TRUE and the conditional formatting will be applied.
Named ranges for simple syntax
If you name the list you are searching (C1:C10 in this case) with a named range, the formula is simpler to read and understand:
=COUNTIF(list,A1)=0
This works because named ranges are automatically absolute.
Case-sensitive version
If you need a case sensitive count, you can use a formula like this:
=SUMPRODUCT((--EXACT(A1,list)))=0
The EXACT function performs a case-sensitive evaluation and SUMPRODUCT tallies the result. As with the COUNTIF, this formula will return when the result is zero. Because the test is case-sensitive, “apple” will show as missing even if “Apple” or “APPLE” appears in the second list. See this page for a more detailed explanation.
Explanation
Conditional formatting is evaluated for each cell in the range, relative to the upper left cell in the selection. In this case, the formula uses the MOD function to check the remainder of dividing the value in each cell, with the value in cell E2, which is 9. When the remainder is zero, we know that the value is an even multiple of the number 9, so the formula checks the result of MOD against zero.
When MOD returns zero, the expression returns TRUE and the conditional formatting applies. When MOD returns any other result, the expression returns FALSE conditional formatting is not applied.
Note E2 is input as an absolute address ($E$2) to prevent the reference from changing as the conditional formatting rule is evaluated across the selection B4:G11.
