[ggm]

The intersection of both. I did a very bad job of plumbing an electric shower into my flat in York back in the 80s and working out how to route the pipe to meet both ends requirement of where I could connect it into supply, and where it had to be to deliver water to the heater, was massively confusing. I am sure a plumber would understand this innately, but I wasted 2+ m of copper pipe trying to "route" it, without understanding the limits of how I could bend it, or cut and use a fitting.

The building code(s) only get you so far. There's also aesthetics.

[bialpio]

Ah, well, I only did PEX and ABS for my reno, very idiot-proof so I managed. ; )

[ggm]

I made the mistake of watching (as a kid) a plumber use his own body to form the curves, and assumed "it's that easy" without taking in that he was 25 years past his apprenticeship and had bent thousands of pipes, and used a former almost all the time. It's a different game when its you, the pipe, and twenty needle-jets of water streaming out of the badly fitted olives, lack of PTFE (or too much) tools which don't fit, pipe with ugly crinkles in them, splits along the length...