Two things you need to know:
1) Systemd boots towards the target given by "default.target". This is typically a symbolic link to the actual target file.
2) Systemd keeps it's targets in /lib/systemd/system and /etc/systemd/system. A file in /etc/systemd/system takes precedence over those shipped with the OS in /lib/systemd/system -- the intent is that /etc/systemd is used by systems administrators and /lib/systemd is used by distributions.
Debian as-shipped boots towards the graphical target. You can see this yourself:
$ ls -l /etc/systemd/system/default.target ... No such file or directory $ ls -l /lib/systemd/system/default.target ... /lib/systemd/system/default.target -> graphical.target
So to boot towards the multiuser target all you need do is to put in own target:
$ cd /etc/systemd/system/ $ sudo ln -s /lib/systemd/system/multi-user.target default.target
