EXFO FTB-2 - Manual (Page 374)

IEEE 488.2 and Specific Command Reference
364 FTB-2 and FTB-2 Pro
Specific Commands
When the data is returned in ASCII, the
<Length> is the number of significant digits to
be returned. A <Length> value of zero indicates
that the device selects the number of significant
digits to be returned.
When the data is returned in PACKED <Type>,
the <Length> is not used and always returns 0.
Example(s) FORM? returns ASCII,6
FORM? returns PACKED,0
See Also FORMat[:DATA]?
:FORMat[:DATA]?