What exactly is the distinction here? What qualifies as a ‘position’ for an a/c to be put into this category?
A recent ADS-B position message or multilateration position result.
So old messages don’t count toward A/C with positions? How old does the message have to be before the aircraft doesn’t count towards this category? And if it doesn’t have a position, how am I still able to track it? Thanks for the help.
A position in the last 60 seconds. The source code is the canonical place to look for this type of thing: https://github.com/flightaware/dump1090/blob/master/public_html/script.js#L1010
Aircraft with a non-ADS-B Mode S transponder send messages that you can receive, but they don’t include position information. The position can sometimes be multilaterated (think “GPS in reverse”) without ADS-B given enough receivers and suitable conditions, but this is not always possible.