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'start' will start the boot cycle (aka, boot into GOS)
As for the other options:
'bar code' will display a bar code with your IMEI, SKU, and Serial Number
'Restart bootloader' will restart fastboot
'Power off' will power off the device, testing it, it also takes you out of fastboot once you boot up again. So technically it can also be used to go back to GOS
'Rescue mode' is for repairs and troubleshooting.
'Recovery mode' is for the recovery screen, usually used to wipe a device without its pincode or sideload an update
But those won't really do much for you, just thought I would mention what those options are