For IDATA?, the response data is: 0 to 999 E-6, 1.00 E-3 to 9.99 E-3, 10.0 E-3 to 99.9 E-3, 100 E-3 to 999 E-3.
For the details, read the GPIB/RS-232C interface operation manual of TOS9200 series (chapter of Message common to all tests).