This TruthValue identifies whether the interface a cryptomap set is attached to is a tunnel interface, such as a GRE or PPTP tunnel, per its description, as opposed to a physical interface. It matters because IPsec applied to a virtual tunnel interface behaves differently for routing and multicast support than IPsec applied directly to a physical outside interface. An admin troubleshooting why dynamic routing protocols work over one encrypted link but not another might find the working one reports true here (a tunnel interface) while the other reports false (a plain physical interface, which can't carry routing protocol multicast/broadcast traffic through IPsec alone).