The corresponding bit is set to 1 to start the operation.
The various bit positions are:
BIT 1: clear interface counters
BIT 2: clear pvc packet counters
BIT 3: clear port ATM counters
BIT 4: clear pvc ATM counters
BIT 5: clear DHCP counters
BIT 6: clear IGMP counters
BIT 7: clear IGMP port counters