Question: what is the difference between a cell reference of b7 and a cell reference of $b$7?
Answer: The other two cell references are still relative references and change as the formula is copied down. ... What this means is that the reference to cell B7 needs only an absolute row reference for this formula to work. As you see below, B$7 is now the cell reference and row 7 will not change when you copy the formula down.