Result columns (not placeholders) can use the \"_multi_iri\" and \"_multi\" suffixes to hold multiple concatenated values produced by group_concat. Use \"_multi_iri\" when the values are URIs (space-separated) and \"_multi\" for literals (newline-separated). For example:

(group_concat(str(?item); separator=\" \") as ?items_multi_iri) for IRI lists.

(group_concat(?text; separator=\"\\n\") as ?texts_multi) for literal lists.