Hi Roma,
dealtCurrency is an optional extension to the FxSingleLeg model, allowing the message producer to express their view on which is the “dealt” currency in an FX Spot/Forward trade. The element takes enumerated values (ExchangedCurrency1, ExchangedCurrency2), so as to restrict the choice to the currencies exchanged in the trade.
As far as the concept is relevant to a FX Swap, then I agree that the values on the near and far legs would be inverted with respect to each other, so as to reference the *same* currency on each leg.
As far as I am aware, no official guidance was published on the use of dealtCurrency in Fx Swaps – this may be an omission in the specification.
Best regards,
Harry