'calspair' unit type

I'm working on a stylesheet enhancement to add support for <imageobjectco>
using the 'calspair' unit type to the DocBook XSL stylesheets, and I have a
question involving the proper interpration of this unit type.  The Def.
Guide reads:

"The coordinates are expressed using the semantics of the CALS graphic
attributes. The format of the coordinates is "x1,y1 x2,y2". This identifies
a rectangle with the lower-left corner at (x1,y1) and the upper-right corner
at (x2,y2). The X and Y coordinates are integers in the range 0 to 10000;
they express a percentage of the total distance from 0.00 to 100.00%."

My question is, where is the point (0,0)?  I started out by calculating
everything from the top-left corner, but further consideration got me to
wondering if the bottom-left corner was correct.

Jeff Beal
Tools Specialist
ANSYS, Inc.
(724) 514-3150