You probably know that books cataloged as a series can be given a sequence number to designate what order they should be sent in, and that series without an order can all be given sequence 0. But here are a few details you might not know:
1. Unordered series (all titles have sequence 0) allowed series to essentially be "first-available." In a Duplication service model, though, everything is available! In that case, unordered series are sent by Publication Date, oldest to newest.
2. You can add sequence 0 titles to an ordered series to send them out after a patron has read the ordered titles. This comes in handy for those long series-writers who also pepper in novellas and holiday specials which might go in-between previous books or not have an intended order at all. (If you aren't familiar with this situation, have a look at Janet Evanovich's books for an example.)
Cataloging these "extra" titles as 0 means that readers who are out of "main series" titles get to read bonus material until the new book comes out, and you never have to go back through all of them to shuffle them further back in the sequence.
