Upward longwave radiation alignment in CPL

[LeeJace]

Dear all,
I am currently trying to figure out the upward longwave radiation flux between CAM and CLM module.
Here is what I found in the cpl folder
call state_setexport(exportState, 'Fall_lwup', bounds, input=lnd2atm_inst%eflx_lwrad_out_grc, minus=.true., rc=rc)
if (ChkErr(rc,__LINE__,u_FILE_u)) return
call state_getimport(importState, 'Faxa_lwdn', bounds, output=atm2lnd_inst%forc_lwrad_not_downscaled_grc, rc=rc)
if (ChkErr(rc,__LINE__,u_FILE_u)) return

call state_getfldptr(exportState, 'Faxa_lwdn' , fldptr=fldptr_lwdn , rc=rc)
if (ChkErr(rc,__LINE__,u_FILE_u)) return
call state_getfldptr(importState, 'Faxx_lwup', fldptr=fldptr_lwup, rc=rc)
if (ChkErr(rc,__LINE__,u_FILE_u)) return

Then I dived deeper into the drivers of cime, and I found

integer :: index_x2a_Faxx_lwup...
index_x2a_Faxx_lwup = mct_aVect_indexRA(x2a_a, 'Faxx_lwup')...
nf = f_hlwupbudg_dataL(nf,ic,ip) = budg_dataL(nf,ic,ip) + ca_a * x2a_a%rAttr(index_x2a_Faxx_lwup, n)
integer :: index_l2x_Fall_lwup...
index_l2x_Fall_lwup = mct_aVect_indexRA(l2x_l, 'Fall_lwup')...
nf = f_hlwupbudg_dataL(nf,ic,ip) = budg_dataL(nf,ic,ip) + ca_l * l2x_l%rAttr(index_l2x_Fall_lwup, n)

But I still can't find the direct linkage between x2a_Faxx_lwup and l2x_Fall_lwup, I would expect there are some assignment operation or threshold error diagnosis.
Could you please kindly guide me how to find there detailed relation in CPL?
Warm regards,
Lee