The card reader has:

[new item at the end]

\item A prioritized list of servers which are capable of handling a
transaction.

\end{enumerate}

[....]

The card reader keeps a list of servers in order to provide some mechanism for
fault tolerance.  This list should be in some sort or preferential order.  In
the case of food service, for example, the central food server would be first
on the list, followed by a backup server on the local subnet.  If the card
reader fails to contact all of the servers on its list, then it has no choice
but to inform the user that his operation cannot be completed and to eject his
card.