What is the difference between an open center and a closed center hydraulic system?
In an open center system no pressure builds in the circuit until a spool is moved; the pump circulates the oil freely. In a closed center system the circuit is permanently under pressure and generally uses a variable-displacement pump with a pressure regulator. Most older and classic tractors are open center designs.
Open center: The pump delivers a fixed flow. While the control valve is in neutral the oil passes through the valve and returns to tank, so pressure stays low. When a spool is pulled the path closes and pressure rises. The design is simple and low cost; on the other hand, because the pump runs continuously, some power is spent even in neutral and the oil heats up.
Closed center: With the valve in neutral the oil path is closed and the line waits under pressure. A variable-displacement (load-sensing) pump is normally used: when there is no demand the pump reduces its flow. It is better suited to running several services at the same time and independently of each other.
This distinction does not directly affect kit selection. The LFT20 and LFT54 do not connect to the hydraulic line; they mechanically pull the cable-controlled Lift-O-Matic lever. Therefore whether your system is open or closed center does not determine how the kit works. The only decisive condition is that the tractor has a cable-pulled Lift-O-Matic mechanism.
Full coverage of the topic: Converting a tractor hydraulic lever to electronic control