Both SPARC based and x86 based systems boot with a boot archive, which is a file system image that contains the files required for booting. When booting from a ZFS root file system, the path names of both the boot archive and the kernel file are resolved in the root file system that is selected for booting.
Booting from a ZFS file system differs from booting from a UFS file system because with ZFS, a device specifier identifies a storage pool, not a single root file system. A storage pool can contain multiple bootable ZFS root file systems. When booting from ZFS, you must specify a boot device and a root file system within the pool that was identified by the boot device.
By default, the file system selected for booting is the one identified by the pool's bootfs property. This default selection can be overridden by specifying an alternate bootable file system that is included in the boot –Z command on a SPARC system or by selecting an alternate boot device from the BIOS on an x86 based system.