The function takes a sequence, or more typically an expression that evaluates to a sequence, and indicates that the result sequence may be returned in any order.
Sequence
The following example is from the XQuery standard and it illustrates how to find pairs of books that have different titles but the same set of authors (possibly in a different order).
<bib> { for $book1 in document("bib.xml")/bib/book, $book2 in document("bib.xml")/bib/book where $book1/title > $book2/title and $book1/author = $book2/author return <book-pair> { $book1/title, $book2/title } </book-pair> } </bib>