Microsoft Excel's Data Validation tool lets you add a drop-down list to a cell based on existing data in a column. However, how this works depends on whether the source data is part of a formatted ...

I have used the Excel ... formula, meaning it was carried over from the formula entered in cell K2. If you wish to modify the formula for this customer, you will need to make those changes in cell K2.

If you've used data validation in Excel before, you probably selected the relevant cells, clicked "Data Validation" in the Data tab on the ribbon, and created the rule in the Data Validation dialog ...