
The particle data is stored in a separate texture if any of the dupli objects uses particle info nodes in shaders. To map dupli objects onto particles the store an additional particle_index value, which is different from the simple dupli object index (only visible particles, also works for particle dupli groups mode). Some simple use cases on the code.blender.org blog: http://code.blender.org/index.php/2012/05/particle-info-node/