This is a bitmask you write to put one or more individual DS0 channels on this port into loopback, where the low bit corresponds to DS0 1 and any set bit puts that channel into loop. Because it operates per-DS0 rather than on the whole port, this lets an admin isolate a single troublesome 64 Kbps channel within a T1/E1 for testing without taking the other 23 or 31 channels on this port out of service. Writing a value of 0x00000005 here would loop up DS0 channels 1 and 3 on this port while leaving all other channels passing traffic normally.