At some point during our last game session, I turned to our warlock and said "I expected you to help us with that, sorcerer", and his answer was "I'm not a sorcerer, I'm a warlock". My reply was "warlock, sorcerer, wizard, conjurer, or mage, I don't care, it's all the same", to which he answered, "it's the same for you, who have no understanding of arcane matters".
So, is character class an in-world concept in your campaigns? Could the mightiest sorcerer in your world be in fact a 20th-level wizard? If my oath of vengeance paladin was trained as part of a monastic order, would other people disagree if he referred to himself and other members of his order as warrior monks and tell him that monks are supposed to fight unarmored?
To answer my own question, except for some very specific situations, like druids in AD&D 2e, I never treated character class as an in-world concept. In my own games, a light-armored fighter with a criminal background does not see himself as fundamentally different from someone with levels in the rogue class.
What about your own campaigns?
