In plain English, this is the "digital ID" of a standard or external hard drive. The Story of "The Generic Traveler"
$ lsusb -v Bus 001 Device 009: ID 1234:5678 Smart Charger bDeviceClass 8 Mass Storage bDeviceSubClass 6 SCSI bDeviceProtocol 50 Bulk-only bInterfaceClass 8 Mass Storage bInterfaceSubClass 6 SCSI bInterfaceProtocol 50 Bulk usb class-08 amp-subclass-06 amp-prot-50