Rating (schema.org)
Rating is a schema.org type that describes a single rating or score, for example on a scale of one to five stars, and is usually used inside Review or AggregateRating.
In practice
Rating rarely stands on its own; it is part of larger structures such as Review, where it represents the actual score given by an individual user. bestRating and worstRating are needed whenever the scale differs from one to five; if they are missing, Google assumes those values.