The Init system is the first process (as PID 1) started by a operating system at system startup that start all other service (ie services)
The following are possible init system:
ps -f --pid 1
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 11:38 ? 00:00:07 /usr/lib/systemd/systemd --switched-root --system --deserialize 21
where: systemd is SystemD - Services
when: ansible_service_mgr == "systemd"
Possible values are: