If my account is usually on vacation... how exactly do my books posted as available on my bookshelf move up through the system?  (I realize when it is on vacation no books on my available shelf can be requested.)  Do my books only move up the queue when my account isn't on vacation, or will they still move towards the #1 spot regardless?  How does it work?  thanks

Your books do keep their spot in line even while on hold and will keep moving on up.   

Your books will still be moving up in the queue, even though not requested, because you maintain your RELATIVE position in the line.  So if the line moves forward, your place moves up too.  When you become active (off of hold) you will be located in a position closer to the front than you were when you started.  If you don't become active (still on vacation hold) even when you are in #1 spot, you just hold a place at #1 spot, but are bypassed for #2 person until you activate your account again.

Question answered!  I understand completely, thanks!

It's not really "what is my place in line" but "when did I post my books" that matters. Each request that is submitted goes to the oldest active book in the system.

Exactly, it's based on the date your book was posted. Oldest book posted = first ordered.

Just want to note--placing a book on your bookshelf on individual hold (as opposed to putting your whole bookshelf on vacation hold) will move your book to the back of the FIFO line.