Part of canonical.launchpad.webapp.metazcml View In Hierarchy
Known subclasses: canonical.launchpad.webapp.metazcml.IFeedsDirective, canonical.launchpad.webapp.metazcml.IMenusDirective, canonical.launchpad.webapp.metazcml.INavigationDirective
For each class in the classes list, found in the specified module, the handler will hookup the class to do something. Since this is a fairly generic mechanism, what that 'something' is isn't important.