Once the initramfs is created, nodes can boot into diskless/diskful/mixed environments by specifying the initramfs on the kernel command line:
The root filesystem itself can be mounted via NFS or from a
local disk. When a portion of the root filesystem has been deployed
on a local disk, the initramfs can be configured to automatically mount
those partitions before
pivoting into the root filesystem.
An entire cluster (or any individual node or group within it)
can be configured any way you want. The default initramfs template can
also be extended to support almost any conceivable
creative boot method.
Note: The standard mkinitramfs utility can still be used to bootstrap
diskful nodes in many scenarios.