While, technically, you can attach review markup to any schema type, for many types displaying star reviews does not add much value for the user. With this change, we’re limiting the pool of schema types that can potentially trigger review rich results in search. Specifically, we’ll only display reviews with those types (and their respective subtypes):

schema.org/Book
schema.org/Course
schema.org/CreativeWorkSeason
schema.org/CreativeWorkSeries
schema.org/Episode
schema.org/Event
schema.org/Game
schema.org/HowTo
schema.org/LocalBusiness
schema.org/MediaObject
schema.org/Movie
schema.org/MusicPlaylist
schema.org/MusicRecording
schema.org/Organization
schema.org/Product
schema.org/Recipe
schema.org/SoftwareApplication