It all comes down to what you are welding. A tack can be anything from a quick zap from a wire machine, think exhaust system, to hold the parts together. Or an actual weld such as needed to hold two pieces of pipe in alignment for welding. Or a short weld to keep two pieces of sheetmetal from warping so far apart that they can't be welded. Or heavy (1") Plate from warping from the heat input. Some tacks can be small and light and some need to be considerably more substancial it all depends on the application and circumstances.