You can apply a full face expression, or make your own mixing the upper and lower face partials. This gives you more possibilities: change your idea on the go, leave the lips that you like and choose different eyes for the expression, use the parts of the expressions that you like more, or even choose similar partials to subtly change the mouth or eyes in an animation, just like what happens in real life.
Natural expressions for real faces, with subtle asymmetries to enhance realism. Good taste expressions, no grimaces. Although they were made especially for Aiko 6, they also work on Genesis 2 Female(s), and figures based on her.