config.xml size param unit definition?

Is there a way to explicitly define the units of the size param on the ImageTarget element in config.xml? something like... Or are the units arbitrary - based on the division of the target region by the dimension values provided? And if so, does this also establish the scene unit? This is the wording in the dev guide that I'm confused about.. [QUOTE][B]Target size[/B] Target size is the actual size of the Image Target in 3D scene units. A developer must specify this in the config.xml file. The config.xml file is generated by the Target Management system, but it can be modified by the developer. By default, the Target Management system defines the size of an Image Target based on its size in pixels, which may not fit the 3D scene. The size parameter is very important, as the pose information returned during tracking will be in the same scale. For example, if your Image Target is 16 units wide, moving the camera from the left border of the target to the right border of the target will change the returned position by 16 units along the x-axis. (x,y) - size of the target in scene units measured along the horizontal and vertical axes of the rectangular target.[/QUOTE] The guide later states that the ImageTarget samples are defined in millimeters, which suggests that the unit is established by the dimension values provided. Also the passage above implies that if no size param is defined, that the system will default to pixel units - is this accurate? And lastly is there a copy of qcar_config.xsd around? I haven't been able to find a schema definition for config.xml.