Epson GT-6500 EN - Manual (Page 180)

Information byte:
This byte has meaning only when the Valid bit is set to 0 and
indicates the difference between the transfer length or
assigned length specified with a command and the actual data
length required by the scanner. The value entered here is
calculated using the following formula.
Information byte = transfer length - actual data length
(assigned length)
(The data byte length is 0 when a negative value is specified.)
Sense keys:
The Initiator can determine the causes of scanner errors by
monitoring the sense keys.
A-18 Interface Specifications