Tgt Forc Crv

Key Tgt Forc Crv in Yield Curve Fxb refers to an optional forecasting yield curve associated with the target currency.
Expects an object of type Yield Curve.

This curve is used for forecasting the future fixings of the target index and is built using interest rate market instruments of the target currency.
Note the difference between this exogenous target curve input and the curve being produced:
Although both curves pertain to the target currency, the former is linked to the target interest rate market, while the latter is linked to traded fx contracts between the target and source currencies.
These two curves are allowed to differ since they are used for different purposes.
The situation is analogous to dually bootstrapped curves where the existence of collateralized and uncollateralized ibor linked contracts gives rise to two different ibor curves.
If omitted, the output yield curve reproduces the input market rates when it is used to both forecast the rates and discount the cash flows of the target leg.

Note:
In the latter case the bootstrapping will fail if the swaps' target legs are par - i.e. carrying no spread and having multiplier equal to 1 - because their NPV will always equal 0 due to the initial and terminal capital exchange.