Test load is to simulate the voltage needed for a specific diode and the resistor is there so you can read the current passing through. The voltage pretty much depends on the current too. If you are using 3 diodes on the test load at lower currents (the voltage will be lower than if you are using 3 diodes at high current) so one setting can work for reds and 445nm it all depends on the current. That's why you need a selectable test load. You need to the read the voltage on the test load to be close to the diodes forward voltage at specific current.
I use two multimeters for that reason, one attached for reading the voltage and one across the resistor for the current. It's way easier that way.