Front-Right Side of Evaporator
This is one of the last stages to making the syrup. You can see that there are four compartments to this part of the evaporator. On the outside, in the middle you may notice that there is a part of pipe in the shape of a U. This is where the back two compartments drain into the front two. The sap is boiled in each compartment; it is then drained out of the front compartment once the syrup is thick enough.
The tap that lets the sap out of these compartments can be moved to wherever there is the most sap at the time. Sometimes they may only use the back compartment, or the back two. Sometimes maybe the front two will be used and so on. The main thing is that the tap can be put wherever it is needed to drain from whichever of the four compartments that has the sap/syrup in it or them.