Looking at the Publisher User Guide 2.0, I find the following:
If a book’s title has one of the following problems, it will removed from sale.
But, when I look at the FAQs in the iTunes Connect "Contact Us" section, I find:
Other metadata fields in iTunes Producer such as Subtitle, Imprint, Series Name, Number Within Series, and Related Products are optional.
It sure would be nice if the left hand knew what the right hand is doing…
PS: Yes, I submitted a bug report to Apple about this.
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