CellGrouping is an option for Notebook which specifies how cells in the notebook should be assembled into groups.
The default setting is typically CellGrouping->Automatic.
With CellGrouping->Automatic, cells are automatically grouped in a hierarchical way based on their styles.
With CellGrouping->Manual, cells must be grouped manually, either by setting up explicit CellGroupData expressions, or by using the Group Cells menu item in the notebook front end.
See Section 2.11.2 and Section 2.11.13.
See also: CellGroupData.
New in Version 3.