Hey, I have been working on a project to design a Phasor Measurement Unit(PMU) in Simulink. And one of the steps involves taking the first harmonic from the FFT block output array(The FFT block itself takes a phase signal as input). I am having trouble figuring out which element in the output array corresponds to the first Harmonic component.

I obtained a simulink file from the internet which uses the element u(2,1,1) from the FFT output array as the First Harmonic array.A screenshot of the block in question.. If I am reading this correctly this denotes the the element in the 2nd row of the 1st column of the array(No idea what the Third 1 if for?).

My question to all this, how to understand that the element in u(2,1,1) denotes the first harmonic component?