Well I just took the most conservative approach, the axis is labelled with DBSM so one unit is assumed to be one DBSM.

The graphs are so important because its one of the rare cases where a US document quantifies the RCS difference of different bands. Hence we can use exclusively US sources to quantify the effect of X-band compared to VHF-band. As you know Russia, China and recently Iran seem to be convinced that their VHF-band radars are effective against US low-RCS systems.

But as you said, we are not sure what one of those units really mean, Hpasp's guess could also be right.