There is some confusion over the use of the term 'frontal area.'
The proper terminology used by all automotive aerodynamicists is 'projected frontal area.'
All automotive coefficients of aerodynamic drag are based upon a vehicles projected frontal area.
The area is taken by the shadow of the vehicle,cast upon a screen directly behind the vehicle,produced by parallel light,as sunlight.It includes all of the vehicle and has nothing at all to do with the 'front' of the vehicle.The use of this criteria is merely a convention,making all coefficients of aerodynamic drag based upon a common metric.
The shadow may coincide with the vehicles maximum cross-sectional area,but not necessarily,as with the Morelli shape,which has a unconventional side and underfloor camber.
Here's a graphic.
