This defines a special committed-bit allowance specifically for the edge case where a circuit has positive Bc credit before a frame arrives but would go negative after accepting it -- a grace allowance letting that one frame through as committed traffic rather than immediately reclassifying part of it as excess. An admin fine-tuning congestion behavior for large-frame protocols, where a single frame could easily straddle the Bc boundary, would adjust cktDeltaBc to control how generous that grace allowance is, with the default of 65,528 bits rarely needing to be lowered except on very tightly-provisioned circuits. For example, a circuit carrying large Ethernet-bridged frames near its Bc limit relies on cktDeltaBc to avoid having a single large frame arbitrarily split between committed and excess treatment mid-frame.