@steve you were talking about whether activities you receive are "in" your inbox.
It's hard to say "where" a digital object is. In ActivityPub, it's very reasonable to say that the object only truly exists at the URL of its id, and everything else is a copy or reference.
But Collection objects can have JSON objects (copies) and/or URLs (references) in the items array. Saying those objects aren't really "in" the Collection is misleading. Using RDF triples to prove that the aren't is wrong.