Line capacity depends on diameter, length and pressure.The fact that a pool heater needs a special HP line if true perhaps has more to do with the pipe run length?
Normal residential pressure is either low pressure (0.25 to 0.5 psi) or higher pressure (2 psi).
Appliances usually use low pressure, so if you use 2 psi, you need a regulator near the appliance to reduce the pressure to the appliance pressure as specified by the manufacturer.