Quantum DLT 4000 Product Manual page 237

DLT4000 SCSI Interface
Number of Blocks
This field will be sent as zero, indicating that all of the remaining logical blocks on
the tape will have the medium characteristics specified by the block descriptor.
Block Length
This specifies the length in bytes of each logical block transferred over the SCSI bus.
A block length of zero indicates that the length is variable (as specified in the I/O
command). Any other value indicates the number of bytes per block that will be used
for read, write, and verify type commands that specify a "Fixed" bit of 1 (fixed block
mode).
DLT4000/DLT4500/DLT4700 Cartridge Tape Subsystem 8-77