It's mentioned in the sysctl man page under the --system option.
--system
Load settings from all system configuration files. Files are
read from directories in the following list in given order from top to
bottom. Once a file of a given filename is loaded, any
file of the same name in subsequent directories is ignored.
/run/sysctl.d/*.conf
/etc/sysctl.d/*.conf
/usr/local/lib/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
/lib/sysctl.d/*.conf
/etc/sysctl.conf
