Meaning of XML attributes for Dia UML objects

[Wangyuan Zhang <zhangwangyuan gmail com>]

Hi Folks, 

I am trying to understand the dia output XML file. What are the meanings of attributes for Dia UML objects. Someone asked this question before, but there was no answer. 
For example, the following shows some tags for a simple Class object. I'd like to know what obj_pos, obj_bb, etc means. Is there any documentation for them ?

    <dia:object type="UML - Class" version="0" id="O0">
      <dia:attribute name="obj_pos">
        <dia:point val="14.5,7"/>
      </dia:attribute>
      <dia:attribute name="obj_bb">
        <dia:rectangle val="14.45,6.95;17.4025,9.25"/>
      </dia:attribute>
      <dia:attribute name="elem_corner">
        <dia:point val="14.5,7"/>
      </dia:attribute>
      <dia:attribute name="elem_width">
        <dia:real val="2.8525"/>
      </dia:attribute>
      <dia:attribute name="elem_height">
        <dia:real val="2.1999999999999997"/>
      </dia:attribute>
      <dia:attribute name="name">
        <dia:string>#Class#</dia:string>
      </dia:attribute>

Thanks
--Wangyuan