Hi Magnus,
These "entity usage aspects" are described here: https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/master/clien...
I'm not sure what you mean with "item id of the page". Which page? eu_page_id is the page id where information from a Wikidata entity is used. eu_entity_id is that Wikidata entity id.
Best Thiemo