Controlling Cell Grouping
Typical stylesheets make
Mathematica automatically organize notebook cells into convenient groups. You can override this behavior, or control it in any way, using menu items, option settings, or from within a program.
(
Ctrl+G) — group together selected cells
(
Ctrl+U) — ungroup selected cells
switch off automatic cell grouping
CellGrouping — whether to do automatic grouping of cells in a notebook
CellGroupingRules — precedence and grouping type for cells
ShowGroupOpener — whether to control cell groups with

CellGroup — symbolic representation of a cell group within a notebook expression