Inversion is shown with arabic figured-bass-style superscript figures attached to the numeral, using the same closed vocabulary as figured bass

A roman numeral's inversion is indicated with the identical abbreviated arabic-figure vocabulary as figured bass — 6, 6/4, 7, 6/5, 4/3, 4/2 — printed as a superscript stack immediately after the numeral.

An inversion figure outside the closed set (e.g. a nonstandard combination, since root position needs no figure at all) is authored.

the vocabulary is deliberately identical to figured-bass-standard-abbreviations above.

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