I cannot for the life of me find a definition of what the Java VM flag CMSClassUnloadingEnabled actually does, other than some very fuzzy high-level definitions such as “gets rid of your PermGen problems” (which it doesn’t, btw).
I have looked on Sun’s/Oracle’s site, and even the options list doesn’t actually say what it does.
Based upon the name of the flag, I’m guessing that the CMS Garbage Collector doesn’t by default unload classes, and this flag turns it on – but I can’t be sure.