Originally Posted by chaley
The column is used for both the name of the series and the number of the book within the series. To see how the information is displayed, select a book, press E to edit metadata for that book, then put something in both the series and the series index boxes. Press OK and you will see how the series column displays the information.
And one of the most useful things about this is that you can then use a metadata plugboard to automatically prefix the book's title with the series name and number when you send the book to your reading device. This is an amazingly useful capability.