BIC have released a statement regarding best practice for use of the title and subtitle metadata fields saying:
Some publishers and other metadata providers are using the subtitle, and sometimes the title fields, in metadata feeds to carry marketing and promotional text. By this we mean using phrases such as ‘Sunday Times Best Seller’, ‘Gripping read from…’, ‘The Richard & Judy Book Club thriller 2017’, ‘”The best thriller writer alive” Another Author’, ‘Man Booker prize winner’ and so on. It is important for discoverability, good customer experience and an efficient data supply chain that these data fields reflect only the true title and subtitle text that appears on the title page. The valuable promotional text should be included in separate and dedicated promotional text fields, and all metadata recipients, including wholesalers and retailers, should be using these fields appropriately.