This is a running counter: total number of registration requests sent by the mobile node. This does not include deregistrations (those with Lifetime equal to zero). 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.