When you set a value to a symbol, that value will be used for the symbol for the entire Mathematica session. Since symbols no longer in use can introduce unexpected errors ...

Automatic Numbering creates an automatic numbering object, or counter, in the current notebook.

Grouping opens a submenu to control cell grouping and toggle cells open or closed.

Initialization Cell makes a cell auto-evaluate whenever the notebook is opened and the kernel is launched.

CellOpen is an option for Cell that specifies whether the contents of a cell should be explicitly displayed.

Locked is an attribute that, once assigned, prevents modification of any attributes of a symbol.

ShowCellBracket is an option for Cell that specifies whether to display the bracket that indicates the extent of the cell.

ShowPageBreaks is a notebook option that specifies whether to indicate in the on-screen display of a notebook where page breaks would occur if the notebook were printed.

ShowSpecialCharacters is an option for Style and Cell that specifies whether to replace \[Name], \:nnnn, etc. by explicit special characters.

SingleLetterItalics is an option for Cell that specifies whether single-letter names should be displayed in italics.