"DU" refers to distribution uniformity. It is the same calculation we use in the industry for irrigation audits.
You're probably familiar with the cup method where water is collected in cups placed across the surface and then a lower quartile distribution uniformity analysis is performed? It is common for looking at the distribution performance of sprinkler heads. We do that calculation for every set of data for moisture and EC so the user can quickly see how consistent moisture and salinity are. You can essentially do an irrigation or nutrient uniformity audit in minutes instead of hours.
"80%" is the magic number. Anything below that indicates a problem. Above that means conditions are uniform. The goal is to achieve above 80% AND hit the target levels with visual insight.
It takes the measurement for total number of samples (minimum of 4 samples needed to calculate DU, ideally 9 or more) and looks at the total average and the lowest 25% average. It's a standard calculation in irrigation auditing and a BMP for GCSAA turf management guidelines. You get to 80% or more by maintaining consistent moisture and EC across the whole zone—improving nozzle performance and set up, hand watering patterns, foliar app consistency, granular app patterns, etc.