what is defined using the Tunnel-Private-Group-ID (81) RADIUS attribute?

When you implement IBNS (802.1x authentication), what is defined using the Tunnel-Private-Group-ID (81) RADIUS attribute?

A.
the EAP type

B.
the shared secret key

C.
the ACL name

D.
the VLAN name

E.
the NAP

F.
the NAF