Hi Runyu,
Can you elaborate on your question? I'm not sure exactly what you're asking.
FSNT is the net shortwave (visible) radiation at the top of the model, and FLNT is the net longwave (infrared) at the top of the model. Combined they give the net radiative balance of the system; that is, if their difference is non-zero the system is out of balance and would tend to warm/cool to re-establish equilibrium.