Hi Faruk,
1. For expense line items the BA will get from its cost center.
2. For inventory accounts, you have to map the plant to BA or plant/division to BA in OMJ7. Based on the plant setting the system will capture the BA in all inventory accounts and also capture in SD transactions
3. For vendor/customer transactions, make business area field as mandatory.
4. For automatic account assignments, maintain the settings in OKB9 based on BA.
Regards,
Mukthar