CellGroupData

CellGroupData[{cell₁, cell₂, ...}]
is a low-level construct that represents an open group of cells in a notebook.

CellGroupData[{cell₁, cell₂, ...}, 1]
represents a cell group in which only the first cell is open.

CellGroupData[{cell₁, cell₂, ...}, {i₁, i₂, ...}]
represents a cell group with cells at positions i₁, i₂, ... open.

Cell groups are typically opened and closed by double-clicking their cell brackets or opener icons.

When cells are entered into a notebook, they are automatically placed in groups unless CellGrouping->Manual is set.

Create a notebook that has automatic cell grouping:

Create a document with manually grouped cells:

Create a group with only the second cell open:

Create a group with its first and third cells open: