When a device carries a SYSTEMD_MACHINE_TAG udev property, tag it for systemd and pull in systemd-machine-tag@<tag>.service, which adds the given tag to the host's machine tags. In other words, attaching the property to a device makes the host auto-tag itself with the specified machine tag.
Files in this directory contain configuration for systemd-udevd.service, a daemon that manages symlinks to device nodes, permissions of devices nodes, emits device events for userspace, and renames network interfaces. See man:udev(7) for an overview of the configuration file format, and man:systemd-udevd.service(8) for a description of service itself. Use 'systemd-analyze cat-config udev/rules.d' to display the effective config.