A modulating/tonicized passage gets a bracket spanning its roman numerals, labeled with the new key

When analysis identifies an extended tonicization or modulation (not just a single applied chord), a horizontal bracket is drawn spanning the roman numerals of the affected passage, with the new key relative to the home key printed at the bracket.

A multi-chord tonicization is labeled chord-by-chord with individual applied-chord slashes with no overarching bracket, making a sustained key area visually indistinguishable from a sequence of unrelated single applied chords, or a bracket is drawn but mislabeled with the wrong key.

Standard convention in harmony-pedagogy textbook engraving (Aldwell and Schachter's key-area brackets); no Gould/Read/Ross equivalent since this is analytical rather than performance notation.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.