matchbox serves iPXE, GRUB, or Pixiecore boot 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. The "first boot" Ignition config if fetched and CoreOS is installed.
CoreOS 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.