Runlevel determines the set of components to be run in normal state. It is a decimal digit from ‘0’ to ‘9’ or letter ‘S’. Traditionally, runlevels are assigned as follows:
Single user mode.
Multiuser with X11.
Additionally, three special runlevels ‘a’, ‘b’ and ‘c’ can be used to start on-demand components without actually changing the runlevel. Once started, on-demand components persist through eventual runlevel changes.