Result columns (not placeholders) can use the "_multi_iri", "_multi", and "_multi_val" suffixes to hold multiple concatenated values produced by group_concat:

_multi_iri — all values are URIs, separated by any whitespace (spaces, newlines, or tabs). Example: (group_concat(str(?item); separator=" ") as ?items_multi_iri)

_multi — all values are literals, separated by newlines, with escaping. Example: (group_concat(replace(replace(?text, "\\\\", "\\\\\\\\"), "[\r\n]+", "\\\\n"); separator="\n") as ?texts_multi)

_multi_val — values can be a mix of URIs and literals, separated by newlines, with escaping. Each value is checked individually: URIs (matching https?://) are rendered as links, literals are rendered as text or sanitized HTML. Example: (group_concat(replace(replace(str(?val), "\\\\", "\\\\\\\\"), "[\r\n]+", "\\\\n"); separator="\n") as ?values_multi_val)