The kernel initramfs only mounts the root partition - parsing fstab and running mount is handled as part of init. This means that an init system needs to suppor

The kernel initramfs only mounts the root partition - parsing fstab and running mount is handled as part of init. This means that an init system needs to support split-usr by ensuring it can go without the contents of /usr at least long enough to bootstrap (which requires special treatment of the mounting scripts/units) or else avoid using /usr at all