Result of " fix ID group-ID ave/correlate Nevery Nrepeat Nfreq: " command

Dear All

Can someone explain me what the components represent in the result of "“fix ID group-ID ave/correlate Nevery Nrepeat Nfreq:” command ?

For example the components of the fulx f_JJ[3]…f_JJ[4]. and f_JJ[5] are used to calculate the thermal conductivity in each direction but I don’t understand
what the f_JJ[1] and f_JJ[2] represent ?. What is I want to average only in one direction? Below is sample calculation of Argon’s thermal conductivity.
compute flux all heat/flux myKE myPE myStress

From the ave/correlate manual (

"The first column has the time delta (in timesteps) between the pairs of input values used to calculate the correlation, as described above. The 2nd column has the number of samples contributing to the correlation average, as described above."