I have seen on a link connecting two highways a reference of the form “15;126” to signify highway 15 linking to 126. Is this standard practice?
Well, there is a kind of definition about http://wiki.openstreetmap.org/wiki/Semi-colon_value_separator in the OSM wiki, but I am not sure whether this fits in your spotted situation.
Maybe a better way to define the destination of a link way can be http://wiki.openstreetmap.org/wiki/Key:destination ?
Thanks, that was very helpful. It definitely shows that the ref tag is being incorrectly used. Combining what you provided with the information at:
http://wiki.openstreetmap.org/wiki/Link_(highway) and
http://wiki.openstreetmap.org/wiki/Tag:highway%3Dmotorway_link
made it all clear for me. It looks like the ref should be removed and an exit_to tag should be used instead.