Commercial airlines publish their flight schedules months in advance through a common industry service, primarily to allow easier cross-airline ticket purchasing, however not all airlines do this (mostly budget airlines in smaller markets, since it costs them fees to do so). The “AirlineFlightSchedules” function returns just data from that service.
Flights can include various types of purposes: commercial airline, cargo, military, general aviation, air taxi, unscheduled service, etc. Only the commercial airline flights (and sometimes high volume cargo carriers) that choose to are listed in the ArlineFlightSchedules function.
If you are interested in tracking the actual status of flights, then our live tracking data can give you a slightly different view, but not as far into the future. When the flight is a few hours from departure, they will actually file the official flight plan with the ATC of the relevant country. FlightAware has data feeds with some countries that allow us to receive these official ATC flight plans. We also have other airline feeds that allow us to fall-back to an unofficial (non-ATC) version of the flight’s times. Additionally, we have an ADS-B network that allows us to estimate departure and arrival times based on the aircraft’s position.
General Aviation aircraft can choose to be blocked, excluding them from public tracking but optionally allowing private tracking to authorized representatives. Military aircraft are generally also excluded by default, upon request from those countries.
If you are more specific in what you’re actually trying to do, I can tell you what is possible.