If you have a SCSI device, such as a CD jukebox, that supports more than one LUN (Logical Unit Number), and only one LUN is detected, you can say yes here to force the SCSI driver to probe for multiple LUNs. A SCSI device with multiple LUNs acts logically like multiple SCSI devices. The vast majority of SCSI devices have only one LUN, and so most people can say no here. The max_luns boot/module parameter allows you to override this setting.