By default, client invoicing will invoice in the currency for which the charge rates have been set. You can check this by running any time/cost report from your reports page. For example, if your project currency for project A is USD, then all time/cost entries will be recorded in USD, and therefore your invoices will always be raised in USD. There may be situations however where you wish to invoice in a different currency. As an example, you may have a client who requires invoices in USD, but you wish to record your own costs in GBP for certain projects for this client.
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To enforce a particular currency to be used in client invoicing, edit the client record, and select a currency for the 'Force invoice currency' field. If you select a currency here, the you must have a defined exchange rate for the currency conversion, otherwise the invoice will not be converted automatically. By enforcing a client invoice currency, the invoices for this client are always raised in your desired currency for any project currency as long as there is an exchange rate defined.