[Zope] Why initializeBasesPhase1 and 2

I noticed that in some products' __init__.py module there's a call to initializeBasesPhase1 and 2 to register a base class as a Zclass. That's true in CMFCore, CMFDefault, CMFPlone for example and not in DCWorkflow and Localizer. in newer cases, createZClassForBase is used instead. So, why is it necessary to register a Zclass for some products and not for others ? TIA -- Andre PZP Enthusiast