Hi,
It depends on the requirement.
If you want to use the currency – This would be country specific second currency which is used in countries with high inflation to improve the value of transcation. You define hard currency for the country that your company assigned to. It is the foreign privileged curr that can be used to do international payments. The config of hard curr is done at country level. When the document is posted the values are updated in loc curr and hard curr.
Group currency 30 is maintained at the client level - this is applicable to all the company codes in the client .Global company currency will be effective to all those company codes which are assigned to your Group Company.group currency is the Currency that the holding (Company) usually uses to do the Consolidation balance among all company codes.
Group currencies are used to enable cross-company postings in controlling for company codes that use different company code currencies.