Hi Josef
Business areas are assigned to Sales Org / Plant /Division etc in SPRO > Enterprise Structure > Assignmengt
Apart from this, you assign them in masters like CC, Assets, etc
Based on this, the expense that you post to these CO objects, or the transactions posted to Plant etc derive the Business area automatically
If you have New GL with doc splitting, the Business Area from Expense flows auto to Balance sheet item
If you wanna default a Busn Area to a recon account, i guess substitution is the only option
Br. Ajay M