[Zope] testing for folders

I am trying to traverse all children of a folder and find all users in all userfolders. I have discovered the following attributes: isAUserFolder isAnObjectManager isPrincipiaFolderish Which I guess were created for this type of task. But I am not sure what the difference is between 'isAnObjectManager' and 'isPrincipiaFolderish'. Could someone explain when I should use each of them. Thanks Tom