These .conf files define filesystem operations that are needed to setup paths. This is commonly creating specific files and directories with specific permissions and ownership prior to running a system daemon. For example an upstart job with:
pre-start script mkdir -p /run/dbus chown messagebus:messagebus /run/dbus mkdir -p /var/lib/dbus end script
Can be replaced with a tmpfiles.d file with:
d /run/dbus 0755 messagebus messagebus d /var/lib/dbus/ 0755 root root
This file should have the .conf extension and be installed to /usr/lib/tmpfiles.d For more information about the conf format see the upstream documentation.