Cross-staff beaming (a beam whose notes visually span two staves) is not representable in Gradus's schema at all

In keyboard writing where a passage crosses between the two hands' staves, a beam is sometimes drawn spanning both staves to show the notes are one continuous rhythmic figure even though they're drawn on different staves.

N/A as an authorable fault — this is a capability gap, since Gradus's curriculum (species counterpoint, chorale, fugue) essentially never needs true cross-staff beaming, and the schema provides no field to declare a note's stave assignment independent of its voice's assigned stave.

Gould, Behind Bars, ch. Beaming — cross-staff beaming conventions for keyboard/harp writing.

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