When the book shows up as an automatic WL selection, why does the book I posted require 48 hours for the member to confirm they still want it? I thought it "automatically" was good to go. Do I not understand the system as well as I thought I did?

It's a bit counterintuitive, I agree!  Basically, when you put a book on your WL, you are just saying that you'd like to have the book someday.  Ideally, when the book becomes available, the person responds "yes!  I want the book" and you can send it.  However, that doesn't always happen.  Sometimes people don't have any credits, so they can't order the book, or are away from their computer for a few days so they miss the email offering the book, or (unfortunately) are bad members who joined and then ran. 

Well, there are two options that a person has to choose from when they put a book on their WL....Auto Request and 48 hour hold...sometimes people choose to put certain books on 48 hour hold because perhaps they don't always have alot of credits or perhaps they aren't totally sure they want that book or what I believe happens most often is they have multiple versions (HC, PB, multiple ISBNs) of that book on their WL and they don't want to Auto Request every version.

If the book was coming up as being on auto request WL, then what probably happened is that the person did not have a credit and the site switched over to the 48 hour hold to allow them time to get a credit and then request the book.  At least that's what I think probably happened.

Or they don't put it on auto request because they'd just rather not.  I have none of my WLs on AR--I'd just rather get the system mail and go accept it myself.  I DO want the books.  Just because I have it on 48 hour doesn't mean I want it less than someone who has it on AR. I just have a personal prefernce for 48 hour hold.

On SaCD if you don't have enough credits and it's on AR it skips you and offers it to the next person, but leaves you on the WL.  I'd rather be given the option of buying/transfering credits.  Since this site is going to go to the same WL as SaCD has, I presume it'll have that feature.  Which while I can see being useful, I don't want to use. So they stay on 48 hour for me.

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"...what probably happened is that the person did not have a credit and the site switched over to the 48 hour hold to allow them time to get a credit and then request the book..."

I think Heather nailed it :-)