VV Approx

Key VV Approx in Fx Vol Spec refers to an optional boolean that determines whether a certain approximation is applied in the formula that calculates the interpolated vol.
The default is FALSE, which means that no such approximation is applied and the vol is given by the equation (12) in the paper below.
If set to TRUE, the approximation is applied and the vol is given by the equation (11) in the paper below.
Web reference available here