Bob, I use the term "loop" because the line /water leaves the manifold and makes its way through the coils and back to the manifold. A "zone" one or more "loops" off a common manifold and controlled by a thermostat. In my shop, there are two zones: one has 4 loops and one has 3 loops. The 4 looper covers 1400 sq ft and the 3 looper covers 1000 sq ft.
Before signing the contract for the system, I spent several evenings on the net where there is a ton of info on various systems. One thing that was common was copper is definitely the best tube for circulating the water: flexible, sturdy, and the best heat transfer rating PEC is the 2nd choice and the only reason I used it was the terrible price of copper tubing. Using copper would have almost doubled the cost of the system.
The pressure testing of the loops was a local code item, but I would have insisted on it anyway because finding a leak would be a bear and then the repair is not something I would want to do or have done.