Unloads modules during the shutdown process. Modules are unloaded in reverse order to when their StartupModule() FINISHES. The practical implication of this is that if module A depends on another module B, and A loads B during A's StartupModule, that B will actually get Unloaded after A during shutdown. This allows A's ShutdownModule() call to still reference module B. You can manually unload a module yourself which will change this ordering, but be careful as you may be unloading another module's dependency!