Wet boom refers to typically stainless pipe that has holes drilled in it for saddle mount nozzle bodies.

whereas most farmer sprayer have boom mounted nozzles that have lengths of rubber hose between the nozzles.

typically converting a “dry” boom setup to wet boom, and getting rid of all of those pieces of rubber hose between nozzles greatly diminishes the chances for herbicide contamination, except it would seem in older John Deere sprayers, they had something funky going on there that made valor cleanout of them especially difficult.