Impl Vols as Spreads

Key Impl Vols as Spreads in Vol Curve refers to an optional Boolean that is used only if Sparse Conv Mkt Vols? = true or Sparse Impl Vols? = true, in which case it determines how the produced vols are represented.
When TRUE, the non-ATM vols are represented as spreads between the absolute vol values and the corresponding ATM values.
When FALSE, the non-ATM vols are represented as absolute vol values.
If omitted, default is FALSE