Unique-key processing allows you to process only the first record of a group. The group is defined by one or more records with the same set of key values. Processing the first record implies that the records you receive will have unique keys.
When you use unique-key processing, you can only read the file sequentially. The key fields are sorted according to the specified sort sequence (SRTSEQ) and language identifier (LANGID).
If you specify unique-key processing, and the file actually has duplicate keys, you receive only a single record for each group of records with the same key value.