sdd:Selection
A sdd:Selection is the intent materialized by select "<SPARQL>" — the object of a sdd:Group's sdd:intent when membership isn't expressible as sdd:AllChildren's single root-plus-predicate shape. Its sdd:select holds the raw SPARQL SELECT string, unparsed and unwalked for embedded IRIs, the way @query query strings are treated elsewhere.
A sdd:Selection is the escape hatch: it exists for membership conditions the fixed intent grammar can't express. That generality has a cost — the query has no distinguished root resource, so a sdd:Selection group has no natural subject. A grounded title still requires an explicit `subject <curie-or-iri>` line on the group; without one, the group can only carry aesthetic content.