Figure combinations must match the closed set of standard abbreviations and their implied chord

Figured bass uses a small fixed vocabulary. The canonical closed set (sorted-descending numeral tuple -> implied chord/inversion) is: [] (no figures) -> root-position triad (5/3 implied); [6] -> first-inversion triad; [6,4] -> second-inversion triad; [7] or [7,5,3] -> root-position seventh; [6,5] -> first-inversion seventh; [4,3] -> second-inversion seventh; [4,2] or [2] -> third-inversion seventh. Suspension figures are a separate voice-leading category, not chord-quality tuples: 4-3, 7-6, 9-8, and 2-3 (bass suspension) denote a dissonance resolving to the adjacent consonant figure.

A figure array like [8,6] or [5,2] (combinations not matching any entry in the closed vocabulary above) is authored, likely indicating a typo or a misunderstanding of which figures imply which inversion.

Read (the standard abbreviation table); Gould; both agree on this core set, though Gould notes some 20th-century pedagogical simplifications Gradus should not adopt given it teaches historical/common-practice figured bass per the Fux/chorale tradition.

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