Define Accounts for Exchange Rate Differences
In this activity you define the numbers of the revenue and expense accounts to which realized exchange rate differences (gains and losses) are posted when open items are cleared.
Note
You can also define the accounts for valuating
open items here. Note however, that you can only make the necessary settings
for these accounts when entering the settings for the closing procedures. For
more information on this, see the activity 
"Prepare Automatic Postings for Foreign Currency Valuation" in
the General Ledger Accounting Implementation Guide.
However the main purpose of this activity, is to define the accounts for realized exchange rate differences since if you do not do so, you cannot post foreign currency documents.
You can differentiate the accounts by currency in which case exchange rate gains and exchange rate losses are posted to separate accounts. Following the first valuation run, you can no longer make changes to your accounts for the valuation posting (since if you did so, you would not be able to reverse valuation postings.)