Each SCSI controller can have a number of buses, each bus can serve a number of devices, and each device can have one or more Logical Units.
The dashes act as wildcards. "- - -" simply means "scan all buses, all devices and all LUNs". The string "0 - -" would mean "scan all devices and LUNs on bus 0", while "- - 0" would mean "scan LUN 0 on all devices on all buses".
If you try to reference a non-existent bus/device/LUN, echo will return "write error: Invalid argument".