You need to do this before you create a volume.
.Pp
Device specific options must come before corresponding devices, e.g.
-.Dl bcachefs format --group=ssd /dev/sda --label=hdd /dev/sdb
+.Dl bcachefs format --label=ssd /dev/sda --label=hdd /dev/sdb
.Bl -tag -width Ds
.It Fl b , Fl -block Ns = Ns Ar size
block size, in bytes (e.g. 4k)