What is the function of the diode in the TL431 equivalent diagram?

[likephysics]

The TL431 is a widely used as reference for voltage regulation.
But in some cases there is a R and a C in the TL431 loop. I googled for the euivalent diagram, its an opamp with a reference and a diode in the feedback loop(+ve terminal of opamp and Cathode).
I don't understand the function of the diode in the feedback loop and across the output transistor (across Anode/Cathode).

 

[AlephZero]

I would guess they are to protect against reverse polarity connections, or transient conditions at switch on and off which could take the "ref" voltage outside the range of the anode-cathode voltage.