ResidualLength indicates the difference between the requested data transfer length in the IORB and the actual number of bytes transferred by the target. This field must always be set to as a non-negative number. The specific error condition Overrun vs Underrun should be indicated by setting the appropriate error code in AdapterErrorCode. If the ADD driver is able to return an accurate ResidualLength, it must set the STATUS_RESIDUAL_VALID bit in the Flags field.