http://schema.org/Rating

http://schema.org/itemReviewed is attached to AggregateRating and Review. I can't see how you'd indicate a simple (and non-aggregate) Rating without packaging it in a Review, unless we add:

Rating domainIncludes itemReviewed.

See also:

• Description of Rating is inadequate #140 (closed in favour of this issue)
• itemReviewed inverseOf review #161 itemReviewed inverseOf review