The TrakMaster output LED meter measures in dBFS, where 0dBFS is the maximum permissible level before clipping. 0dBFS, on the Trak Master, is equal to +20dBu (the more common measurement of signal level in the analogue domain), so when the -20dB LED is lit you are in fact pushing 0dBu through the unit, and when the -10dB LED is lit you are pushing through +10dBu.
The reason for manufacturing the unit like this is that the Trak Master has an optional digital output card for people who wish to connect to a SPDIF input on the receiving device. Knowing where the signal is going to clip is extremely important so as to avoid overloading the converter and causing digital distortion (which should be avoided at all costs).
Some receiving devices may have a lower maximum input level than the Trak Masters +20dBu. In this case, what seems a good level on the Trak Master output meter, may clip the input of the receiving device. It is best to set the output fader on the Trak Master so that the signal is peaking about 6dB below maximum on the receiving device. Once you have set this you may see a good level on the receiving device but an apparently low level on the Trak Master meter - note that this is not a problem.
Use the 'SIG LED' to guide you when setting the input gain on the Trak Master. When the LED is constantly lit (when a signal is present) the gain is set correctly.
If the O/L LED starts coming on for more than a flicker when the signal is loud then you need to back off the input gain a little. Set the output fader as described above to achieve the best signal to noise ratio.