This is what actually distinguishes a channelized-port trap from an ordinary port trap: the specific wavelength or sub-channel number affected. The explicit convention that 0 means not channelized lets the admin immediately tell from this one field whether the alarm concerns a whole physical port or just one channel riding on it. For example, a value of 12 would mean only channel 12 on that port, perhaps one wavelength in a WDM system, is affected, leaving the port's other channels unaffected, whereas a 0 would mean the alarm concerns the whole non-channelized port.