+=item B<-f fmt> | B<--format fmt>
+
+Select the output format for the appliance. Possible formats are:
+
+=over 4
+
+=item cpio
+
+A Linux initramfs. This is the default.
+
+In this case you have to supply names for the C<kernel>
+and C<initrd>, where the C<initrd> is the appliance.
+
+=item ext2
+
+An ext2 filesystem.
+
+In this case you have to supply names for the C<kernel>, a small
+C<initrd> which is used just to locate the appliance, and the
+C<appliance> (the ext2 filesystem).
+
+=item checksum
+
+Output a checksum.
+
+This prints a checksum which only changes when one of the input files
+changes.
+
+You can use this in order to cache the output of a previous run of
+this program: computing the checksum is much quicker than building an
+appliance, and you only need to invalidate the cache (and consequently
+rebuild the appliance) when the checksum changes. Note that the
+host_cpu and the UID of the current user are included in the checksum.
+
+=back
+