Quantum DLT 4000 Product Manual page 281

Count
When spacing over blocks or marks, the Count field is interpreted as follows:
A positive value N causes forward movement over N blocks or marks. The tape is
logically positioned after the Nth block or mark on the EOM/P side.
A zero value causes no change in the logical position.
A negative value -N (two's complement notation) causes reverse movement over
N blocks or marks. The tape is logically positioned on the BOM/P side of the Nth
block or mark.
When spacing to EOD, the Count field is ignored. Forward movement occurs
until the drive encounters EOD. The position is such that a subsequent WRITE
command would append data after the last object that has been written to tape
before EOD.
When executing SPACE, the drive implements the following hierarchy:
Blocks - lowest
Filemarks
EOD
BOM/P or EOM/P - highest
Therefore, SPACE N blocks will halt with GOOD status after the Nth block, or with
CHECK CONDITION status on any occurrence of Filemark, EOD, BOM/P, or
EOM/P. A space N Filemarks will halt on the Nth Filemark or on any occurrence of
EOD, BOM/P, or EOM/P, and so on.
Depending on the size of blocks, read ahead data in the buffer allows some spacing
requests to be satisfied without tape movement.
DLT4000/DLT4500/DLT4700 Cartridge Tape Subsystem 8-121
DLT4000 SCSI Interface