SC3 versus SCSI_3 and

Can someone explain to me the difference between enabling the SC3 flag at the FA/HBA and enabling the SCSI-3 flag on the device?  I understand the commands and syntax behind performing these tasks, and I understand what they're for and how SCSI-3 works.  What I'm trying to understand what is enabled when SC3 is turned on on the Symm, versus what is enabled when SCSI_3 is set on the Symm devices.  I'm looking for the technical detail as it applies to the protocol and the interaction between the host and the devices.  To put it another way, if SC3 is turned on and SCSI_3 isn't what occurs and why, and if the SCSI_3 bit is set on the devices but SC3 is not enabled, same question.  Thanks in advance!