The originally ARC8050DB-NVME daughter board version A USB chip firmware supports single volume (ID0/Lun0) only.

From ARC8050DB-NVME daughter board version B, USB chip firmware can support up to 4 volumes ((ID0/Lun0~Lun3).

The USB chip firmware assign LUN numbers from the lowest available number. The first volume is Lun0; the next volume is Lun1, and so on.