interest calculation

hi sap gurus

while posting interest calculation i am getting problem in maintaining time dependent terms in ob81, when ever i am trying to save it its messaging that the transaction type not maintained.

can any one plz help me in this issue.

2. if no problem canu plz send me the configuration steps for item interest and balance interest calculation with a consept oriented explaination plz.

3.why we r defining reference interest calculation. its just for reference for interest rate or some thing else?

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