Maps areas of sea level rise inundation with high versus low degrees of confidence. In this case, 80% is considered a high degree of confidence—areas that may be correctly mapped as "inundated" more than 8 out of 10 times. Areas with a low degree of confidence represent locations that may be mapped correctly (either as inundated or dry) less than 8 out of 10 times.
Confidence mapping is a fairly complicated procedure that is explained in detail in "Mapping and Portraying Inundation Uncertainty of Bathtub-Type Models" available at http://www.jcronline.org/doi/abs/10.2112/JCOASTRES-D-13-00118.1. In short, the method includes the uncertainty in the LiDAR-derived elevation data (root mean square error, or RMSE) and the uncertainty in the modeled tidal surface from the NOAA VDATUM MHHW (RMSE). This uncertainty is combined and mapped to show that inundation extent is not really a hard line, but rather a zone with greater and lesser chances of getting wet.