Hi Celly
Whole process of currency setting is basically depends on the business process and requirements of your client
1. If your client having one Company with multiple company codes and want to maintain different currency at Com code level then you have to go for Group currency as your company currency which is defined at client level and individual currency at com code level as a legal currency for transaction.
2. If you have one to one relation ie one company=one com code task becomes simple. You can maintain same currency for company and com code.
3. In first scenario you have to maintain Group currency as controlling area currency if controlling area is cross controlling area other wise maintain same currency as co area curr if con area is same as co code
I think this will give some light on your issue