This is a fourth backward-direction traffic descriptor parameter, mirroring circuitFwdTDParam4 in that its specific meaning depends on circuitBwdTDType and isn't explicitly mapped out for every protocol in the source description, most likely serving one of the ATM VBR variants that need a fourth shaping value on the return path. As with its forward counterpart, an admin working with an ATM-VBR backward traffic descriptor would need to consult vendor-specific documentation to confirm exactly what this parameter represents for that type. For example, on a circuitBwdTDType of atm-vbr-ubr, this field may go unused or default to zero since UBR typically doesn't require the same shaping parameters as CBR or VBR-RT.