Technically, run levels were obsoleted with Solaris 10 and the introduction of
smf (service management facility) which supersede the SVR4 init way of handling services.
From an high level point of view, wilestones are somewhat similar to the runlevels. They represent a state where a set of services are started or stopped. Some milestones do match the traditional runlevels, like single-user (run level S), multi-user (run level 2) and multi-user-server (run level 3).
