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

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

A.
the NAP

B.
the ACL name

C.
the VLAN name

D.
the NAF

E.
the shared secret key

F.
the EAP type