Produce results with distinct combinaisons of ?x and ?y (e.g. (a, b) and (b, a) are not distinct, hence (b, a) is rejected). If there are several solutions with (a, b), all such solutions are considered :
distinct::?x c:related distinct::?y
This statement differs from the select distinct ?x ?y where clause because the select clause considers only one solution with (a, b).