Fresnel Equation：what does an observation port exactly mean?

In the tutorial, it says that port2 is an observation port, but what does an observation port exactly mean? 
Notice that in the port mode settings of port2, the norm of E is still the same with port1, both are abs(exp(-i*k*x))==1, but I think it shoud be multiplied with the transmission coefficient t_p, because what port2 receives is a transmission wave whose norm should be smaller than the incident wave from port1.
Can someone help me?
Best regards.