What is TCX?
TCX is Garmin's training format: a GPS track with heart rate, cadence and lap structure wrapped around it. FileSorcery reads it and converts out. Writing one is deliberately not offered, because a schema-valid TCX needs activity timestamps that a converted file cannot honestly invent.
What is GPX?
GPX is the XML that GPS devices and fitness apps speak. A file is usually one recorded track - a long list of points with latitude, longitude and often elevation and a timestamp - plus any waypoints you marked along the way.