The result type returned by the method that is to be converted to the type specified by the RETURNS clause.
If you specify a CAST FROM clause, there must be an existing cast from the “result cast from” data type to the returns data type.
...RETURNS DECIMAL(9,5) CAST FROM FLOAT...
Whenever a LOB that requires data type conversion is passed to a method, the LOB must first be materialized for the conversion to take place.
If data type contains a locator indication, then you cannot specify a CAST FROM clause for the method.