EXFO FTB-2 - Manual (Page 345)

Data Types
FTB-2 and FTB-2 Pro 335
Applicable Data Types for Output —IEEE 488.2

<INDEFINITE LENGTH ARBITRARY BLOCK RESPONSE DATA>
This data type is used by a device to return blocks of 8-bit binary
information when the block length was not predefined or when data
has to be computed later.
Note: If you receive a zero as the first digit (#0), it is necessarily followed by a
<RESPONSE PROGRAM MESSAGE TERMINATOR> so that you will detect
the end of the <INDEFINITE LENGTH ARBITRARY BLOCK RESPONSE
DATA>.
Example: #0<DAB><DAB><DAB><DAB><terminator> where
“<DAB>” stands for data byte.
# 0 <8-bit data
byte>
<RESPONSE MESSAGE
TERMINATOR>