How to read and understand control block diagrams? Need help on understanding OEL control blocks.

I found this explanation on OEL limiters on Kundur Book using control blocks and a graph of field current versus time to explain the overall OEL mechanism. I have been trying to crack this explanation for days, yet I am still struggling. Can anyone explain this to me a little better and how these blocks are linked to make the UEL work as it is supposed to? Are these blocks derived from mathematical equations or just logical statements? Thank you in advance