Hello Manju,
First and foremost, in your generic DataSource based on function modules (getting data from tables FEBKO, FEBEP) you have to sit with your "Relevant Functional Module Consultant" say SD or MM and do extraction in ECC or in BW and asked him for a unique records.
For example, you are going to extract some data/information from SD module tables and there you need help from your SD Functional Module consultant and he will guide/help you to identify a unique record, for instance he will say to you Document No and Billing Date make a unique record. Then you will put document number and billing date containing InfoObjects in your Key Part and rest of the fieds/InfoObject will be in the data part of your Custom/Standard DSO.
Secondly, you have ask him about aggregation behavior for same key records, say you have couple of records with same key, then would you like to aggregate the Key Figures or overwrite them.
I do hope this reply would help you!
Many thanks!
Tariq Ashraf