Check this link
http://help.sap.com/saphelp_nw70/Helpdata/en/74/c047de8e5a49808f28295942dc3297/content.htm
There will be multiple Debit,Credit and balance columns in the base tables for multiple periods in a year (example "XXX01" field means the value for period 01, whereas XXX defines the keyfigure itself . The datsources transposes these columns to rows while extracting.