reserved spares on DVD-RAM

I was looking at DVD-RAM properties in ImgBurn (Windows),
and noticed there is ~120MB reserved space, which are spares.
SCSI MMC-2 shows a spare area in each of 24 zones.
SCSI MMC-4 has information in section 6.27.2.11 on primary and supplementary spares.

I found more useful info he http://www.sharkyforums.com/showthread.php?t=308269

BACKGROUND INFO: DVD-RAM has spare areas used for defect management. This is done by the drive
itself, so there's no way around it. As far as the OS is concerned, a DVD-RAM is exactly like a
hard drive. Any bad sectors are remapped automatically and the OS never knows about them. The
Primary Spare Area (PSA) is always 26 MB and is at the beginning of the media. The Secondary Spare
Area (SSA) is variable-size and is at the end of the media. By default, it's about 100 MB. But the
cool part is, if you think the PSA is enough, you can reformat a DVD-RAM to eliminate the secondary
spare area. To do this:

3. Once you have the media, you want to do a low-level format. To do this, run ImgBurn, and open up
Tools Settings Write. Check the box next to "Prefer Format Without Spare Areas." This sets
ImgBurn to format without a SSA if possible. Then do a full erase. (It calls it a full erase, but
it's actually sending the FORMAT UNIT command to the drive.) After you do this, eject and re-insert
the disc so that Vista recognizes the extra space.

= ImgBurn Disc Info (factory formatted) =

Preferred Format Capacity:
Sectors: 2,236,704
Size: 4,580,769,792 bytes
Time: 497:04:54 (MM:SS:FF)

Maximum Format Capacity:
Sectors: 2,295,072
Size: 4,700,307,456 bytes
Time: 510:02:72 (MM:SS:FF)