IPv6 destination address selection is the process of deciding which IPv6 address a connection should be made to. This is the flip side of IPv6 source address selection, which has been the subject of several earlier articles (start here). Destination address selection is described in the same RFC as source address selection – RFC 6724 (which obsoletes RFC 3484).
If you read the doco (like “man gai.conf”) you would be forgiven for thinking that the contents of /etc/gai.conf controlled source and destination address selection in IPv6. You would be wrong.