There are two types of reservations...
Reservation - A simple queue for who should get the resource next
Advance Booking - Used to book a resource for a particular day in the future
Reserving a Resource
At this point you could tell the borrower that there seems to be at least one copy of the resource available for loan (maybe check the returns trolley) and you would click No to continue. You can continue with the Reservation if you like by clicking Yes.
(The details of the numbered items pointed out are listed below)