StartOfString represents the start of a string for purposes of matching in StringExpression.
Subfactorial[n] gives the number of permutations of n objects that leave no object fixed.
SubscriptBoxOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for selections that specifies settings for SubscriptBox objects.
SubsetEqual[x, y, ...] displays as x \[SubsetEqual] y \[SubsetEqual] ....
Subset
(Built-in Mathematica Symbol) Subset[x, y, ...] displays as x \[Subset] y \[Subset] ....
SubStar
(Built-in Mathematica Symbol) SubStar[expr] displays as expr_*.
SubsuperscriptBoxOptions -> {opt_1 -> val_1, opt_2 -> val_2, ...} is an option for selections that specifies settings for SubsuperscriptBox objects.
SuchThat[x, y] displays as x \[SuchThat] y.
SuperDagger[expr] displays as expr^\[Dagger].
SuperMinus[expr] displays as expr^-.