Cue notes vs. ossia staves — when to use which

A CUE (small notehead borrowed from another part) helps a performer re-enter correctly during a long rest, sitting WITHIN the performer's own stave. An OSSIA is a small alternate stave above/below the main stave offering a different reading of the SAME passage for the SAME performer. A cue answers 'where am I relative to the ensemble'; an ossia answers 'here is another way to play this exact passage.'

Not representable in either Gradus format — no small-notehead/cue mechanism, no secondary-stave-for-alternate-reading mechanism. Directly relevant to Gradus's stated KID-MODE SIMPLIFICATION need: an ossia is the textbook-correct device for 'here's a simpler version of the same passage' without altering the authoritative main stave.

Gould (Behind Bars, cue notes and ossia conventions, explicit statement of the distinction); Ross (practical engraving placement rules).

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