FIFO stands for "first-in, first-out". It is a basic principle of swapping at PBS. 

FIFO as it applies to requests:

• The copy of any ISBN Posted into the system earliest will receive the first request.
• Books "go into line" for requests at PBS as they are posted.
• As copies are mailed out, your book moves forward in the queue
• Any copies posted after your copy will go "in line" behind it.
• Does FIFO mean that titles that are very numerous in PBS aren't worth listing? Definitely not! Some titles are requested very often, and the queues for those books move very fast.

FIFO as it applies to the Wish List:

• The member who put an ISBN on her Wish List first is the first member to whom the book will be offered first when a copy becomes available.
• Wish List queues can move fast, even long ones. It depends on how many copies of that book enter the system, and how quickly they are read and re-posted.

Is the FIFO rule unbreakable?

No. Here are some exceptions:

• If copies "ahead of yours" are declined by the senders (say, because of Requestor Conditions), your copy may be mailed out before those posted earlier.
• You can circumvent FIFO and order a book directly from a member
  • This can only be done by clicking on on her bookshelf
  • this will ONLY WORK if the button actually on the member's bookshelf is the one that is clicked to request the book.
• If you click during the process of requesting a book:
  • FIFO will apply to the first book requested, but does not apply to any additional books you request from that member.
• You can also Post a Wish List book directly to another member's Wish List
  • This can be done only by clicking on her Wish List (and nowhere else).
• If you have Annual Membership with Directed Delivery and you are requesting a book for which numerous copies are available in the system: if there is a "standout swapper" (one who responds quickly to requests and mails promptly, and whose books arrive reliably) geographically near to you, your request will go to that person.

Related Links:

How can I tell how many copies of a book are in the system?
How do I direct my request for a book to a specific member?
Can I offer my Wish Listed book to a specific member?
How to post a book