If the cell is editable, this list of values is used to populate the drop-down System.Windows.Forms.ComboBox used to edit the cell.
If the list starts with a pipe character, then the list is not exclusive and the user may type values that are not on the list.
If the list contains three periods ("..."), then a button is displayed in the cell. When the user presses the button, the grid fires the CellButtonClick event.
If the list contains a pipe followed by three periods ("|..."), then a button is displayed in the cell as before, and the user may also type into the cell.
See also the DataMap property for an alternate way of specifying valid values for a cell.