Hi Josef
1. Country G/L account number in the sense a G/L account which is created under the country COA
2. At the end of the day, Country COA creation is the same as any COA. So, the G/L accounts that you create under a country COA are created in the same way as for a normal COA. If you use FS00 to create an operational COA G/L account say 123, you use the same transaction code to create Country COA G/L account say 456 (In the COA field, you select your country COA instead of operational COA). Once both 123 and 456 are created, you go into G/L account 123, and assign 456 in the alternative account number field in the control data tab. This is how you link G/L accounts under both COA's.
3. The purpose of having 2 different COA's is to report your Financial statements in 2 different formats. In a client, it is prefered that we have a single operational COA for all the company codes so that other aspects like Controlling area do not pose any design concerns. In such a case, you have country COA assigned to each company code as well so that you can report as per the country requirements.
Regards
Sowmya