This read-create object selects the ATM service category (real-time or non-real-time variable bit rate, best-effort UBR, or rate-adaptive ABR) for this traffic descriptor. It governs how the switch schedules and polices the connection's cells, and picking the wrong category for the traffic type causes symptoms like jitter-sensitive voice being scheduled as best-effort UBR and suffering cell delay variation it was never designed to tolerate. A voice trunk misconfigured with ubr(3) instead of vbrRt(4) would compete for bandwidth with bursty data traffic and pick up audible jitter under load.