This is a fourth forward-direction traffic descriptor parameter whose specific meaning depends on circuitFwdTDType, but unlike parameters 1-3 the source description doesn't spell out which value it maps to for each protocol, suggesting it's used only by certain traffic descriptor types (most likely one of the ATM VBR variants that need a fourth parameter such as Sustained Cell Rate or Maximum Burst Size, beyond what CBR or simple CIR/Bc/Be need). An admin configuring an ATM-VBR circuit specifically would need vendor documentation to confirm this field's exact role for that traffic type, since it appears unused for the simpler Ethernet/Frame Relay/ATM-CBR cases already documented in the other three parameters. For example, on a circuitFwdTDType of atm-vbr-rt, this parameter likely carries the Maximum Burst Size that the first three parameters don't otherwise capture.