How to define sub labels for local regions

[HZIH]

Firstly, it is unclear that why the author uses two fully connected layers to train the lower part of the face. Secondly, through observation, we have found that the region "up" is associated with AU1, AU2, AU4, and AU7; the region "mid" is associated with AU6; the region "down1" is associated with AU12, AU14, and AU15; and the region "down2" is associated with AU10, AU17, AU23, and AU24, each sublabel is labeled in binary system. Finally, what is the basis for the division of the "down" region?