De acordo com a página do manual systemd (1) dos diretórios da unidade de sistema:
The systemd system manager reads unit configuration from various
directories. Packages that want to install unit files shall place them
in the directory returned by:
pkg-config systemd –-variable=systemdsystemunitdir
Other directories checked are:
/usr/local/lib/systemd/system
/usr/lib/systemd/system
User configuration always takes precedence.
pkg-config systemd –-variable=systemdsystemconfdir
returns the path of the system configuration directory. Packages
should alter the content of these directories only with the enable and
disable commands of the systemctl(1) tool. Full list of directories is
provided in systemd.unit(5).
Em seguida, para os diretórios da unidade do usuário:
Similar rules apply for the user unit directories. However, here the
XDG Base Directory specification[6] is followed to find units.
Applications should place their unit files in the directory returned
by:
pkg-config systemd –-variable=systemduserunitdir
Global configuration is done in the directory reported by:
pkg-config systemd –-variable=systemduserconfdir
The enable and disable commands of the systemctl(1) tool can handle
both global (i.e. for all users) and private (for one user)
enabling/disabling of units. Full list of directories is provided in
systemd.unit(5).
Observe que essas são as regras do systemd que instruem os empacotadores dos locais apropriados a colocarem arquivos diferentes e cabe ao empacotador seguir corretamente essas regras.