On demand means that the device will only heat when you want it to. This could be activated by drawing (ex. firewood 3), by a button press (ex grasshopper), by using a butane torch (ex Stickybrick JR),or loading with herb when you want to hit it. For the latter, the device would have to be preheated like with many log vapes (ex Underdog). Usually these devices will be full convection. There are some exceptions though, like the vapman.