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Google updated the Review snippet rich results documentation, changing the itemReviewed property to 'One of the valid types', where the valid types are listed underneath.
Previously this value was 'Thing', with the caveat that only the listed types are valid, so functionally nothing has changed here. However it suggests Google were seeing invalid markup where users were reading 'Thing' and not reading the rest, so it serves as a reminder to ensure your Review snippet markup is valid.
Clarified that bestRating and worstRating are recommended properties if you want to specify a different scale than the default 5-point system. Previously they had been listed as required properties, but they were only required if the rating system is not a 5-point scale (which led to confusion).