@elpaso66 hi Alessandro, i have tried your example with "Drill-down (cascading) forms in QGIS Value Relation Widgets" but the results saved in table are always subtype 1 for all type, like:
fid, tipo, subtipo
1, 1 ,1
2, 1, 1
3, 2, 1
4, 2, 1
X, X, 1
can you give me some feedback?