When posting from the goods movement to Financial Accounting, you can influence the setting of business areas of G/L items (if no CO account determination was created) and those of the partner business area.
This is especially useful if the G/L account item in the standard system was created without a business area. From the customer's point of view however, the business area of the material can be entered.
You can customize this in the implemenation guide under "Financial Accounting -> Financial Accounting Global Settings -> Use business transaction events". The process module's number is "2910".
You can find additional information in the activity 
Use business transaction events