Explicit. An individual principal. Identified by principal name. The name does not have to be fully qualified if entered from its default domain.
Implicit. All the NIS+ principals who belong to an NIS+ domain. They are identified by their domain name, preceded by the * symbol and a dot. The operation you select applies to all the members in the group.
Recursive. All the NIS+ principals that are members of another NIS+ group. They are identified by their NIS+ group name, preceded by the @ symbol. The operation you select applies to all the members in the group.
NIS+ groups also accept nonmembers in all three categories: explicit, implicit, and recursive. Nonmembers are principals specifically excluded from a group that they otherwise would be part of.