Key Spot in
Yield Curve Fxf refers to the spot FX rate that applies to the fx forwards, of which the market quotes are entered in Set

Expects an object of type
FX Value.

It must involve the two currencies SRC and TGT, as described at
Yield Curve Fxf
The currency SRC must equal the currency associated with the curve in
Src Disc Crv
The currency TGT must equal the other currency referenced by the market quotes entered in
The ordering in the currency pair (base vs quote) is not important.
The spot rate's settlement date is important and affects the discount factors implied by the produced curve.
This is a consequence of the formula f = sPᵗᵍᵗ/Pˢʳᶜ, as shown at
Yield Curve Fxf
The settlement date is also displayed at _Spot Date