AC 12 can backwards save to 11; from 11 you can backwards save to 10. Any new features in 11 or 12 not in 10 will of course be lost from the model. Most library parts cannot move backwards, so the person in 10 could get a massing model with no lib parts...unless you used just the AC 10 library while working in 12.
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB