The response is made up of a feed element which contains some metadata about the feed followed by a series of entry elements. Each entry is made up of some metadata followed by a content element. Inside the content element is a ContactList element which describes the attributes of a particular contact list. The structure of the Atom elements is described in detail by RFC 4287. The user contact lists are ones created by the user, including "General Interests" list, which is created by default. The system contact lists include "Active", "Removed" and "Do-Not-Mail". You cannot manipulate the system contact lists directly as they simply indicate the status of each contact, but we provide them in the contact list collection to give you an ability to query all active, removed or opted-out contacts.