When a publication listed in the online catalog of a library to which you are subscribed is not currently available for loan, you can place it on hold.
In addition to the mention On hold, a publication's detail page indicates that it is not available for loan by the presence of the button Place a hold (whereas the button Borrow is displayed for available publications).
Tap the cover image of the desired publication.
In the publication's detail page, tap the Place a hold button. A confirmation screen shows the estimated date the publication will become available, and your rank in the queue.
To cancel your hold from this confirmation screen, tap the the Cancel hold button.
When the publication you have placed on hold becomes available, you will receive an email notification.