This read-create Unsigned32, when non-zero, points to a tunnel-data definition that gets added, or encapsulated, onto packets matched by this class map. An admin uses this in scenarios requiring traffic to be tunneled after classification, such as directing a specific traffic class into a GRE-style tunnel for special handling. For example, an admin needing to tunnel a particular class of traffic to a remote analysis or security appliance would set this index to point at the relevant tunnel definition, causing matched packets to be encapsulated accordingly.