This read-write value specifies, in cell-times, the same kind of GCRA policing tolerance as its ABR counterpart, but for the receive cell-stream of a UBR connection when no explicit tolerance was given at setup. Because UBR carries best-effort traffic with no rate guarantee, this tolerance mainly matters for catching genuinely broken or misconfigured senders rather than shaping legitimate traffic, so it's typically left looser than the ABR default; setting it too tight would start policing ordinary UBR bursts that were never meant to be rate-limited. Leaving this value very small could cause bursty best-effort web traffic riding a UBR VC to get needlessly tagged, even though UBR isn't supposed to enforce a rate contract in the first place.