AllowGroupClose

is an option for Cell that specifies whether a cell group can be closed normally.

Details

Closing a group collapses all cells in the group so that only the head cell is visible.
AllowGroupClose should be set in the head cell of the cell group.
With the default setting AllowGroupClose->Automatic, any cell group can be closed normally.

Create a notebook where any cell group can be closed normally:

The cell group can be closed normally:

Create a notebook where the "heading" cell group cannot be closed:

Now the "heading" cell group cannot be closed:

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