Why relation terms (e.g. dcterms:ispartof) are not present in xml-format outputs like, for instance, dcmes-xml and dc-rdf?
Do you mean relations coming from a plugin, like ItemRelations?
Do you have any suggestion to have the relations shown in xml outputs?
I think really the only workable option in the current code is to add a new feed/format that includes them.
The existing formats really don’t offer much in the way of places for plugins to hook into them to extend the output, so there’s not really a way for Item Relations to get the relations to appear there.
A hook or filter for the formats is probably a good idea to allow something like this, but it’s just not currently an option.