Generation Schema is only available for SQL Scripts and PL/SQL, PKB, and PKH file object types, this dropdown list displays the list of products that are present in Stat for the source environment. If a value is selected in this column, the object is generated on the target environment using this schema. If the value is blank, the default APPS schema is used.
Furthermore, generation schema should be available only for given object types and it should not be available once the archive set is in progress to just any object type.