SXR will be a cross reference system for a series of books. When I am reading through a very long series of books, especially when it has been several months since I read the last book in the series, I often forget who some of the smaller characters are, or what some series-specific term means.
I want to be able to look up who the person is or what the term meant. Of course, many people aren't going to be as far into a series as I will, and they may get spoiled by events that are going to happen in the future. To prevent this, I'd like to have a versioning system that will allow you to look up definitions that are only in the text you've read so far.