The parent relationship for a child post is stored using the _variable_ field name _wpcf_belongs_[CPTSLUG]_id where [CPTSLUG] is the slug of the CPT that the parent relationship regards (a CPT can have multiple parents, one parent for each CPT), so if a post has a parent of type authors (authors being the slug of the parent CPT), the relationship would be stored in the field named _wpcf_belongs_authors_id
The field would hold the post ID.