[Zope] What is "default__class_init__"?

Can anybody tell me what you use "default__class_init__" from Globals for? I've seen it used many times in python products but never really understood its use. Is it used in conjunction with setting the "__ac_permissions__" property? If so, using the more modern "AccessControl.ClassSecurityInfo()", is there no need for "default__class_init__" anymore? Curious, Peter