From Xojo Documentation
Used to get or set the state of the cell with the coordinates of row, column (zero-based). This assumes that the passed cell is a CheckBox cell. Use the CellType property to set a cell to a CheckBox cell. CheckBox.VisualStates is an Enum of the CheckBox class.
The CellCheckBoxStateAt method enables you to get or set the value of a tri-state Checkbox cell. Any cell of type TypeCheckbox box can store one of three values: Checked, Unchecked, and Indeterminate.
To set up a cell as TypeCheckbox, use code such as this in the Open event:
The Indeterminate state places a minus sign in the checkbox (macOS) or filled in checkbox (Windows and Linux).