"-o" specifier in DNAT definition?

["Robert P. J. Day" <rpjday@xxxxxxxxxxxxxx>]

  in a book i'm reading discussing (among other things),
setting up DNAT, there is a sample rule:

  # iptables -t nat -A PREROUTING -i eth0 -o eth1
	-d <internal server IP address>
	-j DNAT --to-destination 192.168.0.1

the question: how can the explicit specification of the
"-o" interface affect this?
  
  after all, it's quite possible that the interface that
would normally be chosen to forward to the internal
server would not be the one listed with "-o".  (perhaps
the admin made a mistake.)

  if there is an obvious conflict here, what happens?
does this DNAT just go unsatisfied?

rday