systemd-sysv-generator — Unit generator for SysV init scripts
/usr/lib/systemd/system-generators/systemd-sysv-generator
systemd-sysv-generator is a generator
    that creates wrapper .service units for
    SysV init
    scripts in /etc/init.d/* at boot and when
    configuration of the system manager is reloaded. This allows
    systemd(1)
    to support them similarly to native units.
LSB headers
    in SysV init scripts are interpreted, and the ordering specified
    in the header is turned into dependencies between the generated
    unit and other units. The LSB facilities
    "$remote_fs", "$network",
    "$named", "$portmap",
    "$time" are supported and will be turned into
    dependencies on specific native systemd targets.  See
    systemd.special(7)
    for more details.
Note that compatibility is quite comprehensive but not 100%, for more details see Incompatibilities with SysV.
SysV runlevels have corresponding systemd targets
    (runlevel).
    The wrapper unit that is generated will be wanted by those targets
    which correspond to runlevels for which the script is
    enabled.X.target
systemd does not support SysV scripts as
    part of early boot, so all wrapper units are ordered after
    basic.target.
systemd-sysv-generator implements
    systemd.generator(7).