I still haven't had time to dig out the older AFMs .. but the D-X generally referred to the carpet which was the intersection of corrected TODR against ASDR which was overlaid with either V1/VR or V1/V1b data.
Either this or the separated data presentation could be used with the same convenience to run up computer simulations. The D-X presentation simply allowed a single chart to be used instead of a number of separate charts.