"Fraction"

"Fraction" is a front end token that creates a FractionBox and fills the numerator with the current selection.

This token is equivalent to the menu command Edit Expression Input Fraction.

"Fraction" fills the numerator with the sub-expression to the left of the selection if it is an insertion point.

To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].