RESET WHEN is a Teradata extension to the ANSI SQL standard.
Depending on the evaluation of the specified condition, RESET WHEN determines the group or partition, over which the ordered analytical function operates. If the condition evaluates to TRUE, a new dynamic partition is created inside the specified window partition. To define a partition based on a column reference list, use the PARTITION BY phrase. For more information, see PARTITION BY Phrase.
If there is no RESET WHEN phrase or PARTITION BY phrase, then the entire result set, delivered by the FROM clause, constitutes a single partition, over which the ordered analytical function runs.
You can have different RESET WHEN clauses in the same SELECT list.
A window specification that specifies a RESET WHEN clause must also specify an ORDER BY clause.