From Xojo Documentation
If a cell is editable, a CellAction event occurs when the user finishes editing a cell. Row and Column are zero-based.
“Finishing editing” is defined as exiting the cell after clicking in it. Tabbing out of the editable cell or clicking another cell triggers this event. Clicking a checkbox in a checkbox cell also qualifies as "finishing editing".
The user doesn't necessarily have to change the contents.
This code will display changes made to an editable cell when the user finishes editing (by either pressing return or switching to a different cell):
MessageBox("You entered: " + Me.Cell(row, column))
This event handler is also called when the user clicks a CheckBox cell: