Added in M5b: the share of the partition's rows up to this one, ties included
Added in M5b (PLAN §11.9): the bucket, from 1, of buckets near-equal ones
Added in M5b: (rank - 1) / (rows - 1), from 0 to 1
Added by Cube (PLAN §11.6, Q2): one number per row, ties or not
A window-only function, as saved (spec §10, PLAN §11.6, §11.9): it ranks each row of its partition by the window's sort, so it takes no column and needs a sort. A Group doesn't know them (PLAN §11.5, Q4).