D value relationship is about the size of the particles used in the specification of materials. D meaning Diameter.
In a USGA spec green the principal is a bit like a sponge in the bath....you make the sponge wet hold it up and water passes to a point where water will be suspended against gravity...the perched water table effect...add one more drop of water and one drop will pass out...that is what we try to achieve in a true or old fashioned type USGA spec (87).. I only really ever learned the one method and I have spoken to others that believe the 87 method has never been improved and all recent advancements are more to save costs or to make the installation easier..ie the quality of the 87USGA spec has never been surpassed IMO.
Part of the process is to select the three materials Rootzone, Grit Layer, Gravel that conform not only to certain USGA specs but also have the right diameters. The idea is the Diameter values end up giving you a straight line, or as straight as possible when you plot their values on say graph paper. The perch water table effect will work best when your corresponding layers are between 5 and 10 times its formers size, but with three layers you need to select two of your best materials then marry the right material to the third layer, for instance if you used a 0.35mm sized rootzone, then used a 2mm grit, you would want to find a 12mm gravel...your D-value relationship is 6...if you could only find a 20mm gravel then you would need to find the mid point and balance both values, in this case a 2.5mm grit woud balance pretty well to give you a D-value relationship of 8. What is not good is to use a 0.35 rootzone with a 4mm grit then a 10mm shingle.