The ISTC began development in the early 2000s as a way of collocating editions of textual works. It’s not technically a “Work ID” for books, but was misperceived that way – in actuality, it’s an identifier of text strings. So (broadly speaking) the hardcover, paperback and ebook edition of a book would all receive the same ISTC, but the French translation would not, because the text strings are different.
The ISTC is not necessarily assigned by a publisher, or a library, or a bookseller. It can be, but it doesn’t have to be. There is no “ownership” of the ISTC like there is with other identifiers.Anybody who wants to register a textual work – whether it’s an author, an agent, a publisher, or whoever - must submit a request to an ISTC registration agency with the necessary metadata needed to distinguish that work from others. The registration agency determines whether or not that request qualifies for a new or an existing ISTC.