Reference Document is not a required field.
To avoid repeated entry of the same business transaction to record statistical key figures (SKF), you can use existing SKF actual posting documents as a template for further postings.
To do so, enter the number of the reference document.
You can get the document number of this document by choosing F4 help for the document date or by searching per document date, period or fiscal year.
To display the data for the reference document, choose Display reference document.
To use (and later change) this data, choose Confirm.
If you wish, you can display further information on the posting such as the user or the exchange rate type.
To do so, choose the tab strip Additional information.
The above is from SAP Documentation "Entering Manual Actual Postings" in this link - http://help.sap.com/erp2005_ehp_07/helpdata/en/16/0cd553088f4308e10000000a174cb4/content.htm