Well, "ghost flight" is not a defined term, so it is whatever the airline makes of it. Maintenance for parked aircraft were mostly just traffic patterns, so simply take off, fly around the airport and land. Usually around 15 minutes of flight time. But of course they were sometimes used for other stuff as well. We used them to transport crew to and from training. We are limited to maximum 19 passengers, since that means we do not mean a cabin crew for that or cabin crew on board is not paid for active duty (just deadheading which is cheaper for the company). Since i work for a LCC we never carry cargo to begin with. Friends in other airlines told me of flights with 20 to 40 passsengers in an A350, but a full cargo flight. So that could be justified as "ghost flight", but since the cargo rates are way up it was probably somewhat profitable.