Doubt in RFC 3484

[jiten shah <[e-mail]>]

Consider a topology where two routers are connected back to back. these
link have only a IPv6 link local address. I want to ping a global address
on the other switch through this link. RFC 3484 says that for  selecting
the source address :

 It is RECOMMENDED that the candidate source addresses be the set of
   unicast addresses assigned to the interface that will be used to send
   to the destination.  (The "outgoing" interface.)  On routers, the
   candidate set MAY include unicast addresses assigned to any interface
   that forwards packets, subject to the restrictions described below.

      Discussion:  *The Neighbor Discovery Redirect mechanism [14]
      requires that routers verify that the source address of a packet
      identifies a neighbor before generating a Redirect, so it is
      advantageous for hosts to choose source addresses assigned to the
      outgoing interface.  Implementations that wish to support the use
      of global source addresses assigned to a loopback interface should
      behave as if the loopback interface originates and forwards the
      packet.*

So since in my topology the outgoing link does not have a global v6 address
I need to use address on other interface but I do not understand the above
condition. Can someone simplify it on what basis we can use the addresses
on the other interface ?
Can some one please explain me