> when I was in the Systems Group at Sun we worked with AT&T on their solution which was the whole "run level" thing

Interesting that you mention Sun. Didn't SMF precisely replace runlevels with SMF milestones[1] in much the same way systemd replaces runlevels with target units?

[1] https://docs.oracle.com/cd/E23824_01/html/821-1451/hbrunleve...