The "TGA Systems" menu item organizes your objects according to the object hierarchy that you defined yourself using the object form. Up to the "Building" level, objects are grouped according to the levels in the Property Explorer. Below that, you'll find the systems, and below those, the system components.
See also:
📄 Ein Objekt einem anderen Objekt unterordnen IC1950
📄 Objekttyp zur Anzeige im Objekt-Kataster aktivieren IC2709