Reference key while posting with document type KK

Hi Gurus,

I am making a posting via tcode RERAPP and the accounting document gets posted with a document number. Now, this document is posted with document type KK. When this is done, the reference key field in the document header gets populated with a unique number.

Generally the reference key gets populated with a combination of document number, company code and fiscal year.

But in this case, it is not so. It is populating a different combination.

Is this a standard case, does there exist a logic whereby this is being populated. Can you help me in this.

Best rgds

Satya