Docs may miss an important point on the use of Character Face, that is if you simply connect relation constraint, you’ll not see any available attribute on character face to connect!
How to show up character face attributes…
First of all, we have to create some new custom expression in our character face.
If we drop one more time character face into a relation constraint, we will not see any attribute.
My tip is to make attribute animatable, in that case it will appear in relation constraint! You can make property animatable in for example Properties tab. Use red button on right of a property’s name.
Good luck and happy animating!