Hi Magnus,
These "entity usage aspects" are described here:
https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/master/clie…
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