In the GeoSciML 3.2 Encoding Cookbook for Inspire WFS Services (a great help!) it is stated on page 9 that the relationship shall be encoded with a value from a vocabulary, such as the one under development by the CGI.

So far I can't find any vocabulary for this by CGI, does anyone know the status of this?

I saw that BGS was using

<gsml:relationship
xlink:href="http://dummy.resource.geosciml.org/classifier/cgi/featurerelation/geologicHistoryRelation&quot;/>

in one of their WFS's, which seems lika a good solution for now. We will use this instead of making up our own...