Averaging heat auto correlation function

Dear All

I am calculating thermal conductivity calculations using the sample provided for Argon and I have hesitation how to average
the Heat auto correlation function.

.fix JJ all ave/correlate {Nevery} {Nrepeat} ${Nfreq} &

        c_flux[1] c_flux[2] c_flux[3] type auto file Data.dat ave running

.My question is why/how we choose the 3rd,4th and 5th as average ?

.variable k11 equal trap(f_JJ[3])*${scale}

 variable k22 equal trap(f_JJ[4])*${scale}
variable k33 equal trap(f_JJ[5])*${scale}


" This fix computes a global array of values which can be accessed by variousoutput commands <http://lammps.sandia.gov/doc/Section_howto.html#howto_15>. The values can only be accessed on timesteps that are multiples of/Nfreq/since that is when averaging is performed. The global array has # of rows =/Nrepeat/and # of columns = Npair+2. The first column has the time delta (in timesteps) between the pairs of input values used to calculate the correlation, as described above. The2nd column hasthe number of samples contributing to the correlation average, as described above. The remaining Npair columns are for I,J pairs of the N input values, as determined by the/type/keyword, as described above."