When you connect upstream transformations to a Union transformation, you initialize the output fields. The initial output fields are an exact copy of the input fields in group Input1.
When you define output fields, note the following:
•After you initialize the output fields, you cannot change the output fields by connecting or disconnecting the input group.
•You can manually add output fields if you add them before you connect one of the Union transformation input groups.
•When you add an output field, you define the field name, data type, precision, scale, and optional description. The description can contain up to 4000 characters.
•If you connect the Union transformation to an upstream transformation that does not pass in any fields, the output fields are not initialized.
•At run time, the mapping passes null values to output fields that are not in a field mapping.