Create a Cell Note
You can attach a note to a cell to contain text such as a comment, a question, or any text. A cell with a note attached displays a small red square (cell note indicator) in the upper right corner of the cell, as shown in the following figure.
When the pointer is over the cell note indicator, the cell note text appears.
Use caution in choosing a red color as the background for a cell that could contain a cell note. The cell note indicator could be invisible against a red background.
The cell note indicator does not appear when the cell is in edit mode.