This setting is a boolean setting. It specifies whether or not a floating-point parameter should use exponential interpolation by default. The user can always later change this in the Timeline tool window. The setting defaults to false, so parameters are normally not interpolated exponentially.
If you want your formula to run on earlier versions of Ultra Fractal as well where the exponential setting was not available, test for the VER40 symbol, as shown in the example below.
default: float param ExtraMagnification caption = "Extra Magnification" default = 1 $ifdef VER40 exponential = true $endif endparam