You should use absolute cell references in Excel when you want the formula to refer to a specific cell regardless of where the formula is copied or moved. Here are some common scenarios:

* Constant Values: If your formula uses a fixed value like a tax rate, exchange rate, or conversion factor stored in a specific cell (e.g., cell B10), you want that cell reference to be absolute. This ensures the formula always uses that specific value even when copied to other cells.
* Headers or Titles: Absolute references are useful for referencing headers or titles in your spreadsheet that remain constant across rows or columns. For example, if your formula calculates a total in a specific column and you want the column title (e.g., "Sales" in cell A1) to be included, using an absolute reference for A1 ensures the title stays correct when the formula is copied down the column.
* External References: If your formula links to data in a different worksheet or workbook, absolute references are crucial. This ensures the formula always references the correct external location.

Here's an example:

Imagine you have a table calculating product sales with a commission rate stored in cell B10 (10%). You want a formula in cell C2 to multiply the sales amount in B2 by the commission rate.

- Relative Reference (Incorrect): If you use `=B2*B10`, this creates a relative reference. When you copy the formula down to cell C3 (or any other cell), the reference to B10 will adjust down one row as well (becoming B11). This is incorrect as you still want the commission rate to be 10%.
- Absolute Reference (Correct): By using `=$B$10` (dollar signs before both row and column), you create an absolute reference that locks onto cell B10. Now, when you copy the formula to other cells, the reference to the commission rate will always stay at B10, ensuring accurate calculations.