Apparently they correspond to the screen indices in the xml file:
<screen x="0" y="0" i="0" j="0"/>
<screen x="384" y="0" i="1" j="0"/>
<screen x="0" y="216" i="0" j="1"/>
<screen x="384" y="216" i="1" j="1"/>
I'll add a comment to the doxygen