Underl Swap
Key Underl Swap in FltFlt Swaption refers to the option's underlying interest rate swap.
Expects an object of type IRS.
Assuming Direction = Call, the option holder has the right to buy the swap defined here for a zero strike price.
This means, the option holder has the right to enter into a swap receiving the index associated with the first leg and paying the index associated with the second leg if the underlying swap is of REC type and vice versa if it is of PAY type.
This is reversed if Direction = Put
Only the portion of the swap lying beyond the option's expiry is relevant.
The actual swap being entered upon exercising the option is displayed in _Entered IRS