"...you can use partial geometries that contain only some body parts ..."

The way I read it is that PFE can generate a morph for every actor (body part) if you use a an obj file containing all the  body parts as a source.  If you only want morphs for the head and neck, then you must use an obj that only contains a head and neck. That's how I interpret what you quoted, though I have not tried this in practice.