Why do some book's descriptions have a "Members Wishing" field listing a number (oftentimes 0) and some book descriptions don't have that field at all?

If there are copies of that book posted into the system then that field doesn't exist.  If there are no copies of that book available on PBS, but no one actually wants the book, then the field says 0.  If there are no copies of the book available on PBS and people want the book and there is a queue of people waiting to get the book once it gets posted then the field shows the number of people wanting to get the book.  This is called a wish listed book. 

Ohhh. Got it now. Thanks. 

No problem.