SAP note 2083799 provides a composite scenarios involving posting with Accounting BAPIs. But I want to share with you more details related foreign currencies scenario:
The currencies are maintained in transaction OB22 for each company code. You can also see the 2-digit currency type keys here, which are used in accounting interface as well.
Currency type = 00 (document currency) and Currency type = 10 (local currency) are the only real hard-coded keys. You can also see this in OB22, as you cannot change the currency type key for local currency. For 2nd and 3rd local currency, the value depends on the kind of currency you are using. You can find a complete list of the different currency types in SE11 by checking the value range of the domain CURTP.
You see in currency type that basically it always ends with a 0. So the typical values are 10, 20, 30, 40... You also have a valueation field directly underneath with a single digit. This can indicate legal valuation, profit-center valuation and so on. For FI only legal valuation is relevant. If another valuation is maintained, then this value in valuation field will be added to currency type. So you can have a customizing like this:
2nd LC, Currency Type 30, Valuation 0 – legal valuation, currency USD
3rd LC, currency type 30, valuation 2 – profit-center valuation, currency USD
In this case LC2 would be currency type 30 and LC3 would be currency type 32. Only LC2 would be transferred to FI. LC3 would be relevant for profit-center accounting. Eventhough the amounts in LC3 are not relevant for FI, they are determined in FI-Interface (function FI_DOCUMENT_CHECK) if they have not been passed to accounting interface.
You do not have to provide the local currency amounts to accounting interface. Only the document currency is mandatory. But of course you CAN submit local currency to accounting (as MM does for example) then only the missing local currencies are determined in FI-Interface. Only important fact is: IF you submit at least one local currency, you should submit it in all line items and not just at single items. Otherwise a balance issue can occur.
More information on correct setup of OB22 is listed in note 335608.
I hope this helps!