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.