This sets the ATM Quality of Service class this circuit is provisioned under -- CBR for constant-bit-rate traffic like voice or TDM emulation, or one of three VBR tiers for bursty data -- valid only for ATM-type circuits per the description, and it determines how the switch schedules and polices this circuit's cells. An admin provisioning a voice-over-ATM circuit would set cktAtmQoS to cbr(1) for the strict timing guarantees that traffic needs, while a bursty LAN-interconnection circuit would use one of the vbr classes. For example, setting cktAtmQoS to vbr2(3) on a data circuit allows more burst tolerance than vbr1 while still getting better treatment than best-effort, appropriate for moderately bursty application traffic.