My understanding is the following:
Suppose that There are 3 nodes, node A with addresses IPA1, IPA2,...,
IPAN, node B with IPB and node c with IPC
Now node B is communicating with A. As ULIDs for the communication they
are using IPB and IPAj.
so for the communication between A and B if there is a failure in the
path associated to IPB and IPAj, the communication can be preserved
using alternative locators for node A (assuming both ends support SHIM)
Now, suppose that the application running on A passes as application
level information IPAj to an application running on node C.
At this point, node C only has one of the addresses associated with node
A, IPAj (the one obtained through the referral).
At this point, if IPAj is unreachable from node C, the communication
will fail, because node c has no knowwledge about the alternative
locators for node A.
There are possible solutions for this, like using the reverse tree for
obtaining alternative addresses for node A (this may have some problems
also) or modifying the app so that all addresses are included in the
referral (but this needs changing the app of course)