When it comes to flame rectification, a single-rod (local-sensing) system relies on the flame's ions to conduct a small electrical current that is then detected by the system. This type of system is typically used in smaller heating appliances, such as furnaces.
On the other hand, a dual-rod (remote-sensing) system uses a second rod that is placed outside of the flame to detect the current. This type of system is typically used in larger heating appliances, such as industrial boilers.
Overall, the main difference between these two systems is the location of the sensing rod(s) and how they detect the current.