De acordo com man systemd.unit:
UNIT LOAD PATH Unit files are loaded from a set of paths determined during compilation, described in the two tables below. Unit files found in directories listed earlier override files with the same name in directories lower in the list.
Table 1. Load path when running in system mode (--system). +------------------------+-----------------------------+ |Path | Description | +------------------------+-----------------------------+ |/etc/systemd/system | Local configuration | +------------------------+-----------------------------+ |/run/systemd/system | Runtime units | +------------------------+-----------------------------+ |/usr/lib/systemd/system | Units of installed packages | +------------------------+-----------------------------+Additional units might be loaded into systemd ("linked") from directories not on the unit load path. See the link command for systemctl(1). Also, some units are dynamically created via a systemd.generator(7).
Observe que /lib é apenas um link simbólico para /usr/lib