Domain Validation is part of the SCSI standard. It describes how to negotiate the best possible transfer agreement between two devices. There are three different messages that can be sent:
SDTR (Synchronous Data Transfer Request)
WDTR (Wide Data Transfer Request)
PPR (Parallel Protocol Request)
WDTR is used only to negotiate width (8 or 16 bit).
SDTR can be used to negotiate up to Ultra-2 speeds.
PPR subsumes the capabilities of both WDTR and SDTR and can negotiate speeds of up to Ultra-640. It can also set various parameters, such as Information Units, Quick Arbitration Select and Double Transition clocking.