When a row is created in a temporal table that has a system-time or transaction-time dimension (column), Vantage sets the ending bound of the column to UNTIL_CLOSED and the row is considered open. When the row is closed, Vantage sets the ending bound value to the closing timestamp.
IS UNTIL_CLOSED evaluates to true if the ending bound of the specified column is the maximum timestamp value, 9999-12-31 23:59:59.999999+00:00.