I often see military flights in the sky, which do not appear on FlightRadar24. I do understand that low-flying military airplanes do no necessarily get picked up by these trackers, and that some military aircrafts switch of transponders. However some flight do appear on this ADSB-Tracking website. For example currently you can often see USAF F-16 near the Ukraine with a Tanker in the back on some kind of holding pattern.

What logic or factor governs whether a flight appears on FlightRadar24?