In the context of `group by` it's treated as grouping sets, but that's not its only use. (Though that does seem to be special cased in terms of parsing, since afaict - I can't find the full query BNF on mobile - `grouping sets` is not optional.)
GROUP BY [ ALL | DISTINCT ] grouping_element [, ...] grouping_element can be one of: ( ) expression ( expression [, ...] ) ROLLUP ( { expression | ( expression [, ...] ) } [, ...] ) CUBE ( { expression | ( expression [, ...] ) } [, ...] ) GROUPING SETS ( grouping_element [, ...] )