Mike and I have finally figured the situation out in part due to WmAx pointing me in the right direction too.
is the total excursion in a completely linear fashion, thus no distortion is introduced via the driver moving in a non-linear fashion.
is the total throw of the driver used in a specific application with a certain amount of chosen distortion (chosen by the subwoofer designer). The distortion is caused by the driver moving in a non-linear fashion and is generally not audible if properly implemented. Many manufacturers/designers have a set given amount of distortion that is allowed in a design caused by allowing a driver to have an excursion beyond its xmax.