systemd.user.services.<name>.overrideStrategy

Defines how unit configuration is provided for systemd:

asDropinIfExists creates a unit file when no unit file is provided by the package otherwise it creates a drop-in file named overrides.conf.

asDropin creates a drop-in file named overrides.conf. Mainly needed to define instances for systemd template units (e.g. systemd-nspawn@mycontainer.service).

See also systemd.unit(5).