PI Plus Percentage of Weight Passing a 200 Sieve

We have a qualification in our Native Soil Specification requiring, "Sum of plasticity index when tested in accordance with ASTM D4318 and the percent of material by weight passing a Number 200 sieve shall not exceed 23 when tested in accordance with ASTM C136." It's been in there forever. We had a submittal where they actually tested the native soil on a project, and it exceeded the limit. I looked at it and thought it was a typo. It seemed like adding together apples (PI) and airplanes (percent passing a 200 sieve). But then I saw in an very old textbook (Winterkorn and Fang, foundation Engineering Handbook) a formula::
GI=(F-35)[.2+.005(LL-40)]+.01(F-15(PI-10), where GI is Group Index and F is the percent passing the 200 sieve. It seems to add together PI, LL and F.
My question is, is there a parameter where adding together PI plus the percentage passing a 200 sieve is useful?