They are unique per target and volume you can view this in the test.log, example below shows a LV UUID and the lun x signiture.

LV UUID dssYVw-rjOQ-cyt2-iKuN-oXu1-tetB-Epv9ha


cat /proc/net/iet/volume
*-----------------------------------------------------------------------------*

tid:2 name:iqn.2007-07:dss01.vm
tid:1 name:iqn.2007-07:dss01.boot
lun:0 state:0 iotype:fileio iomode:wb path:/RAMDISK/iSCSI_targets/dssm9F-JQn2-SVp0-FUPI-0q6g-2plO-1ZgmZF/lun
lun:1 state:0 iotype:fileio iomode:wb path:/RAMDISK/iSCSI_targets/dsssTt-yIJU-0jvE-i4JV-EzTn-cetb-nWFFT1/lun