matchbox serves iPXE or GRUB configs via HTTP to machines based on Group selectors (e.g. UUID, MAC, region, etc.) and machine Profiles. Kernel and initrd images are fetched and booted with Ignition to install CoreOS Container Linux. The "first boot" Ignition config if fetched and Container Linux is installed.
Container Linux boots ("first boot" from disk) and runs Ignition to provision its disk with systemd units, files, keys, and more to become a cluster node. Systemd units may fetch metadata from a remote source if needed.