This is a running counter: total number of Registration Requests denied by home agent -- unknown home agent address (Code 136). Since Mobile IP counters like this only increment and never reset on their own, an admin polls it periodically and watches the rate of change rather than the absolute value — a counter that suddenly climbs fast points at a specific failure mode (e.g. bad authentication, a misconfigured agent, or a flapping mobile node) tied to this exact event.