They may have been created using Data|Validation.
Debra Dalgleish has some tips:
http://www.contextures.com/xlDataVal01.html
Patrick wrote:
I received a spreadsheet with a column of cells that are selectable by
clicking a down arrow or white triangle. It selects Y or N. I want to add
N/A. How do I open it? What is the function?
--
Dave Peterson