This column, if not blank, indicates what we have seen as the code indicating the callsign used by the aircraft for this particular movement.
For registered airlines, the callsign will normally begin with a 3-character code issued by the ICAO. Sometimes, however, it will simply begin with the 2-character equivalent issued by IATA.
Private planes will very often use (approximately) either their registration identifier or the letter N followed by their registration number.
The complication comes in, however, because these callsigns are set manually by the crew of the aircraft concerned. Unfortunately, there tend to be errors of all sorts. Sometimes there will be an error such as "EEZY" instead of "EZY", or "TAAP" instead of TAP. Sometimes even a commercial airline will simply use its registration identifier. Sometimes there is a completely meaningless jumble of characters, or even nothing at all. Thus, the way that the program attempts to evaluate the callsign is certain to fail occasionally.
If a selection includes any letters then it is assumed to be a request to specify all aircraft whose callsign starts with the selection.
For a full list of ICAO and corresponding IATA callsigns,, see
http://www.flugzeuginfo.net/table_airlinecodes_airline_en.php?sort=icaoasc