Applied/secondary chords are notated as a slash expression: numeral-of-tonicized-degree over numeral-of-the-key-it-tonicizes

A chord that functions as a dominant (or other applied function) of some scale degree other than the tonic is written as X/y where X is the applied chord's own roman numeral relative to the tonicized key and y is the roman numeral of the scale degree being tonicized (e.g. V/V, vii°7/vi).

An applied dominant is mislabeled with a single numeral relative to the global tonic instead of the slash form, losing the pedagogically important distinction between a local tonicization and a home-key scale-degree chord.

Aldwell and Schachter ch. on tonicization/secondary dominants — the slash notation is the near-universal standard; Dorico's Roman Numeral Analysis implements the identical slash syntax.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.