You’ve probably used an Excel data validation drop down list, where you can click the arrow, and select an item to fill the cell. With a bit of programming, you can customize a drop down list, to select multiple items.
Single Selection Drop Down
By default, data validation drop downs only allow you to select one item.
In the worksheet shown below, you can select a month from the drop down, and it overwrites any previous selection in that cell.

Select Multiple Items From the Drop Down List
Instead of limiting users to a single selection, you can use a bit of programming, combined with the data validation list, and allow multiple selections.
You can display all the selected items across a row, or down a column, or keep them in the same cell.
Keep reading, to see some examples of this technique, and how it works.
There is also a video that shows the multiple selection drop down list. To try it for yourself, follow the download link f
Watch the Data Validation Video
To see these techniques, and a few other multiple selection examples, you can watch this short video.
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How can I change the script to allow this to work on every cell in the column. For example I have the script working for cell I8 how do I make it work for cells I9 onwards?