Fare Classes 101

Until I started counting my PQPs, had no idea that Fare Classes are so important in my life. Yet I am a total newbie. All I've done is activated the expert mode but the letters are making no sense yet.

I got bit hard as I took an Air India flight from Bengaluru to Delhi and another from Delhi to Seoul. To my surprise the Delhi to Seoul flight, for which I purchased a Business Class upgrade gave me about 300 PQP. Upon calling United they explained the math - only original booking class counts and I had discounted economy which is a 50% hit on PQP. Is there a quick way to get wiser about this? I can't even see fare classes on most booking sites until after the tkt is issued. 🤔