Welcome to the Metadata Millennium: A Complete Overview of What Metadata Can Do for Publishers
But supply chain metadata also includes commercial metadata: the price of the book, the date it's available for sale, perhaps things like sales territories and discounts. This also includes a lot of information the recipient (who is going to distribute or sell the book to an end user) needs to know. For physical books, the bookseller needs to know things like how many copies are in a carton, what the dimensions of the book are, how much it weighs. For ebooks, the metadata needs to convey information about file type, file size, and maybe even version.
And of course the publisher wants to include marketing metadata -- that's what helps the book sell. Subject metadata has always been used for physical books to guide the bookstore as to where to shelve the book. For ebooks, subject metadata is even more important: it's what enables users to discover books on subjects they're interested in. But marketing metadata can include a lot more, such as reviews, prizes the book has won, a bio of the author, other books by the author, and other books on the same subject.
There's a lot more metadata that can and should be included -- information about accessibility features, pedagogical information for educational books, etc. But we'll save that for later.
Three Essential Standards: ONIX, BISAC, Thema
All the supply chain metadata is tracked by a publisher's internal databases. That metadata is often in proprietary formats, which is a fancy way of saying "they just made it all up": the vocabularies and codes and database fields are whatever made sense to that publisher, often many years ago, before there was much thought about anybody outside the publishing house needing to understand them. You can imagine what a mess this can create when that metadata, which is often incomplete, inconsistent, and even incomprehensible, is sent out into the world.