if a pin is marked by an InputAttribute containing IsSingle, this will force the pin to hold only one value instead of one instance of the defined type.
Example:
[Input("Width", AutoValidate = false, DefaultValues = new double[](Input("Width", AutoValidate = false, DefaultValues = new double[) {1.0, 1.0, 1.0}, IsSingle = true)]
protected Pin<Vector3D> FWidth;
will create a pin, that has a default of {1,1,1}, but as soon as I connect the pin to an IOBox or reset it with Alt+RightClick, it will restrict the input to only one value (instead of three values).