This is the subaddress that, combined with the node's base address, routes an incoming X.25 call directly to this node's control port, without requiring a separate routing table entry. This is effectively the well-known extension a technician dials to reach the node's management or control terminal remotely over the X.25 network, distinct from any of the data PAD ports also reachable through similar subaddressing. For example, a technician dialing the node's base address plus this control-port subaddress reaches the node's configuration console directly, without needing the node to have a preconfigured route entry for that specific call.