OPA551: OPA551

Druilhe,

1. Before explaining the position, I will discuss the values.  The 1TF capacitor allows for the component to act like an open at DC but a short for all frequencies starting a very low frequencies (e.g. millihertz).  The 1TH inductor allows the inductor to act like a short at DC and an open for all frequencies starting a very low frequencies (e.g. millihertz).
2. The reason for this test circuit is to make the circuit open loop for all frequencies above very low frequencies and closed loop at DC.  The closed loop at DC is required for spice convergence.  If the circuit is not closed loop at DC it will drive to one of the supply rails and not function for AC.   For AC the loop is open.  Since we want to look at open loop gain, loop gain, and 1/beta the circuit needs to be open loop for AC frequencies.
3. Answer for 1/beta:  on page 4 you can see the AC equivalent circuit at the bottom left.  This circuit applies the input signal to the feedback network in an open loop configuration.  Thus, Vfb shows how the output signal is attenuated to the feedback node.  This information is the basis of 1/beta (1/Vfb = 1/beta).  
4. Answer to your question for Aol:  Open loop gain is the output of the op amp divided by the differential input of the op amp.  In this circuit, Aol is Vo/Vfb.
5. Answer for loop gain: Loop gain is Aol x Beta.  Using the factors from items 3 and 4, loop_gain = (Vo/Vfb)(Vfb) = Vo.

best regards,  Art