Payoff Ccy

Key Payoff Ccy in FX Option refers to the optional denomination currency of the net payoff amount assumed to be paid by the option (necessarily regarded as a NDO here, i.e. Non Deliverable Option) on the date Tᴾ specified in Pmt Date
It expects an object of type Currency, which must equal either the base currency BASE or the quote currency QUOTE of the ratio BASE/QUOTE defined in the FX object specified in key FX.
If omitted, the default is that currency that differs from the underlying currency specified in Underl Ccy.

Note the entry here is used only if Tᴾ differs from the option expiry T.
The reason is that if the option's net payoff is paid on T, its denomination currency has no effect on the option's NPV and the entry here is effectively ignored.
But if it is paid at some later time Tᴾ, the payoff is proportional to the payment at T of a zero bond on the currency defined here that matures at Tᴾ.
The PV of that zero bond as of T can be factored out with the net result being that the payment delay from T to Tᴾ introduces a multiplicative factor to the vanilla option price that equals the forward discount factor from T to Tᴾ as implied by the curve associated with the currency here.