Primary Delegate: A signing official (i.e.: delegator) has turned over his/her full authority to a primary delegate. This is a long-term solution for signing officials who are delegating daily responsibility for reviewing+approving proposals to another person. The action requests and the action list reminder email only go to the primary delegate (not the delegator). So for example, if Christine were made Primary Delegate for Cindy, Cindy would not get any action requests or email notifications. They would only go to Christine.
Secondary Delegate: A signing official has designated alternate(s) who are simultaneously active. The signing official can still take actions AND the secondary delegate can as well. Action requests appear in the Action Lists of both the signing official AND the delegate. When either acts on the document, it disappears from both Action Lists. People often have a secondary delegate who takes care of their action requests when they are not available to do so (i.e. back-up approver).