[jmpe]

Ok, dug around in the code.

In Smartmontools I found the code for this variable:

http://smartmontools.sourceforge.net/doxygen/atacmds_8cpp_so...

It's code 179 (0xB3).

From Samsung's website:

http://www.samsung.com/global/business/semiconductor/minisit...

  ID # 179 Used Reserved Block Count (total)

  This attribute represents the number of reserved blocks that have been used as a result of a read, program or erase failure. This value is related to attribute 5 (Reallocated Sector Count) and will vary based on SSD density.

.. so at least for samsung there's use of exact numbers.

From Intel's website:

http://download.intel.com/newsroom/kits/ssd/pdfs/intel_ssd_5...

(Ctrl-F for "Available Reserved Space")

.. they use a normalized value (100).

So it can be either percentage or absolute value, depending on manufacturer.