Irrigation pipes/feed pipes are used to allow water to travel around your growing system. The irrigation pipework that you pick depends on many factors.
The size of the irrigation pipe or irrigation line that you need depends on what you're connecting it to; the diameter of your pipe needs to match that of what you're connecting it to.
If you're doing a completely DIY system, you're best off picking your irrigation pump first (based on the area you want to cover / number of plants you want to grow). You will then need to pick pipe that matches the size of your pump's outlet.
For drippers, you only really want to use delivery pipe, which has a small diameter of 4mm.
Some irrigation pipes do. Look out for double-layered pipes. A white outer layer reflects heat from your grow lights to prevent it from warming your nutrient solution. An inner black layer blocks UV rays. Overall, this type of pipe keeps your nutrient 3oC - 8oC cooler than standard irrigation pipes.