lhs === rhs yields True if the expression lhs is identical to rhs, and yields False otherwise.

SelectedNotebook[] gives the currently selected notebook in the front end.

SpanFromBoth is a symbol that can appear at a particular position in a Grid or related construct to indicate that the corresponding position is occupied by a spanning element ...

SpellingOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for notebooks that specifies settings for spell checking.

SqrtBox[x] is a low-level box construct that represents the displayed object Sqrt[x] in notebook expressions.

StyleData["style"] is a low-level representation of the contents of a style definition cell.StyleData["style", " environment"] represents the contents of a style definition ...

StyleDefinitions is an option for notebooks that gives definitions for the styles that can be used in a notebook.

SubscriptBox[x, y] is the low-level box representation for x_y in notebook expressions.

Subscript[x, y] is an object that formats as x_y. Subscript[x, y_1, y_2, ...] formats as x Subscript[y, 1], Subscript[y, 2], ....

SuperscriptBox[x, y] is the low-level box representation for x^y in notebook expressions.