Comment # 8 on bug 1054473 from Franck Bui

(In reply to Martin Wilck from comment #4)
> IIUC that should have the same effect as "^D" - finish initrd processing,
> switch root, and continue booting. Is that wrong?

"^D" is equivalent to run "systemctl isolate default.target" after the shell
exits.

OTOH "systemctl isolate initrd.target" will start initrd.target and only
services pulled in by it. Basically they're used to setup the root file system
device but nothing more.

So you likely don't want to isolate initrd.target since the system won't switch
to the final rootfs and won't continue to boot the system normally, actually
the system will be stuck.