Piping (tubing vs pipe)

Looking at various cars there seems to be a variety of threaded pipe and flared tubing in the piping systems. I have not been able to detect a pattern. Is there a rhyme and reason to when one is used in preference to the other?

David

David
Typically from the boiler to the first valve is forged steel fittings or Sch 80 pipe. From there on Stanley’s I use 0.049 wall copper tubing, although where more support is needed I use pipe. The steam line to engine is all pipe. Water lines to the main boiler check and all fuel lines are tubing.
Rolly