A placeholder whose parameter name is context is filled automatically with the resource whose page the view is being shown on. Nothing needs to be declared beyond the placeholder itself.

On a resource's own page that is the resource. On the page of one of its parts it is the owning resource, while the view's own target field receives the part; both arrive in the same request. This is what lets a query about one part still find the space that maintains it, without naming that space in the query.

Declare it as an optional multi-value placeholder, ?__context_multi_iri, with its own values block in each scope. A federated service sub-select that uses only this placeholder shares no variable with the outer query and is therefore evaluated once, rather than once per result row.