Here is another, more descriptive quote from John:
"The SS100 is a PT100 but with a separate presence for the clean channel and the option of a very glassy clean channel which makes humbuckers have that Flesh for Fantasy type tone, the swicth takes you from "Glass" to the Old OD clean channel whihc is a tad less low mids than the current clean channel. That just works better for Steve with the Humbuckers"
dc