the starting point for the aggregation group within the partition. The aggregation group end is the current row.
The aggregation group of a row R is a set of rows, defined relative to R in the ordering of the rows within the partition.
If there are no ROWS or ROWS BETWEEN clause, the default aggregation group is ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING.
The default when there is no ROWS clause for FIRST_VALUE/LAST_VALUE is different. For more information, see FIRST_VALUE / LAST_VALUE.