The list of Access Definition allows you to choose no, one or multiple Access Definitions. 

Access definitions come in handy as to when a MarvelClient Administrator does not have the right to create group documents in the public addressbook, or when one does not want to create additional groups in the public addressbook for a particular client management purpose.

New Access Definitions can be created using the "New Access Definition" button at the top of the Access Control tab on the Who tab.

Existing Access Definitions can be accessed from the view [General\Access Definitions].