Function Class Clause
This optional clause specifies the class of function being defined. Specify it only for aggregate functions.
The aggregate function is invoked for each group in a SELECT statement that requires aggregation and computes a single value for each group it is called for. You can specify an aggregate UDF anywhere you can specify a built‑in aggregate function.
A function of one class cannot have the same name as a function of another class within the same database, even if their parameters differ. Overloading function names is only supported within the same class of a function.
This clause is a Teradata extension to the ANSI SQL:2011 standard.