As Forc Curve

Key As Forc Curve of function Roll Curve in Yield Curverefers to an optional boolean that may be used to ensure that the produced curve is suitable for being used to forecast interest rates of which the reset date lies in the future but before the horizon date Tʰ
The default is FALSE, which results in a curve having zero discount factors for all maturities prior to Tʰ and is therefore the correct curve to use for discounting purposes in a rolled pricing context.
Setting it to TRUE results in a curve that retains non-zero discount factors prior to Tʰ
More details at Roll Curve