This selects the traffic descriptor type governing the four circuitBwdTDParam fields for traffic flowing in the backward direction -- 'out of the network' toward the circuit endpoint -- independently of the forward-direction type and independently of the endpoint protocol. Having a separate backward descriptor type lets this circuit be provisioned asymmetrically, for instance guaranteeing a smaller committed rate in the return direction than the forward direction, which is common for asymmetric access services. Setting this to atm-vbr-nrt(6) while the forward type is atm-cbr(4) would give this circuit CBR-guaranteed forward bandwidth but only variable, non-real-time-guaranteed bandwidth on the return path.