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OPDS is the standard at the core of SimplyE. An OPDS feed is a list of books. Each entry in the feed contains metadata for a book (title, author, etc.) and instructions on how to get a copy of the book.
One OPDS feed can link to another, just like one HTML page can link to another.
There are two basic syndication types
Active Draft Specs:
Library Simplified Extension
There are all kinds of reasons why one OPDS feed might link to another, but these are the two most important, and should give you the general idea:
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