the last number is always the bar shift. 0 is the current bar and 1 is the bar before it.
the 2nd parameter from the left (in your example is 2) is the custom indicator index. Every indicator have buffers where the data is saved, so 2 is the 3rd buffer (zero base).
you can see in the code what indicator buffer value you want by looking at that line of code

SetIndexBuffer(0,ExtMapBuffer1);
SetIndexBuffer(1,ExtMapBuffer2);

so, if you want the value of ExtMapBuffer2 you set the iCustom parameter to 1