This controls which person list will be displayed on the checkout form.
When books are checked in without a preceding checkout, they'll be checked out under this person.
Format text to pass to sprintf. See https://secure.php.net/manual/en/function.sprintf.php for more info. The first parameter passed is the collection abbreviation followed by the capture groups of the call number.
The three label line fields describe how to format a label for the spine of a book.