This selects the traffic descriptor type governing how the four circuitFwdTDParam fields should be interpreted for traffic flowing 'into the network' from this circuit's endpoint, and notably this is independent of the endpoint's Layer 2 protocol (circuitEndPoint1Protocol) -- a circuit could run over Ethernet but still use an ATM-style CBR traffic contract, for instance. An admin provisioning quality-of-service or bandwidth guarantees for the forward direction must pick the type here first, since it dictates whether the subsequent parameter fields mean CIR/Bc/Be (for Ethernet or Frame Relay-style shaping) or PCR/SCR-style ATM cell rate parameters. Setting this to atm-cbr(4) means circuitFwdTDParam1 will be read as a Peak Cell Rate value rather than a committed information rate.