I think I know the stage it's talking about and it's a bit in between iirc. There's a few reboots that can happen. One is while the installer is running, which reboots into the disk. Technically there's at least a portion of the registration process that happens during the install boot, but there's also a section that happens after on the first boot where registration gets finalized and all the registration hooks actually happen on the first boot. However if the node can't reach the registration endpoint URL the elemental-system-agent reboots the box.