Any character who belongs to the Barbarian class has the ability to rage, wherein they gain the ability to shrug off wounds that would kill a lesser person as long as that wound is inflicted while they are raging (among other benefits). If that ability isn't tied to their barbarism, then where does it come from? Why can't anyone else do that whenever they get angry? What is the in-game reality which is associated with a character belonging to that class?
Up to the player.
For example, my current 5e PC is a Bar3/War 6. Conceptually, this PC (soldier background) is completely civilised, never worn a loincloth in his life. It's just that he has....anger management issues! They stem from the circumstances of his birth which (briefly and tastefully) involve him being conceived at the same moment his father transformed into a werewolf for the first time! Think Blade from the comics/films, but instead of being a bit vampire-y he's a bit wolf-ey. When he rages, he seems to manifest some minor wolf-ey features (hair, eyes, what-have-you) and the game mechanics of all that are simply the barbarian's Rage without any game mechanic alteration whatsoever! Exactly the same barbarian class mechanics, fluffed as a super-civilised army officer.
My fluff. It's up to me.
Just like I fluffed my background (soldier-officer) into Captain of the Avant Guard, the best regiment in the army, focusing on scouting and trail-blazing, with a uniform so smart its got a PhD from the best university in the land!
Yeah, I fluffed it, not the DM. I imagined this whole regiment. The DM is okay with it.
Sure, the DM
could refuse. If he were a jerk. He
could say that he has already created every regiment in every army in the world and there can be no more....but why would he? Just to be a jerk? As if he has already thought of every detail in the world and the players are not allowed to think of anything? What kind of jerk would be like that? DMs are
happy to let players think about that kind of thing! It's good that the player is so invested!
But doesn't that mean the player can abuse it by saying he's the king? Well, at that point the player is probably treading on the DM's toes, because the DM probably
will have already decided who the king is and even if he hasn't he doesn't want the PC to have that kind of power.
But the player could easily have chosen the Noble background. If the DM has a load of noble families already detailed, what does the DM do? Say that the player doesn't have more money or influence beyond that stated in character creation? Of course, I wouldn't expect anything else. My job as a player is to explain
how I've only got this much money/power/influence despite being a noble. I may be a younger son. It may be that all the money and magic and influence that surrounded me as I grew up doesn't actually belong to me but to my family, and when I run away to join the circus/adventuring company I could take only what the book says I have as starting equipment.
I'll think of something, and it will make sense. Happy to collaborate with the DM; in fact, that collaboration with the DM is the 'A' option for both player and DM. But what DM would say, "No! I'm the DM, you don't get to decide about anything except your own PC, therefore you cannot choose your parent's names, or backgrounds, or....anything except your own PC".
Try it! Try to make a background for
every PC you will ever play without 'treading on the DM's creative toes' and even mentioning an NPC. Who were your parents? No idea, the DM hasn't informed me yet. What kind of jerk DM works that way?
Does the DM say that I can't be a noble on the grounds that he's already created every noble family and knows how many children each noble family has and you are not one of them? Does he give you a list of acceptable noble families? Or does he allow you to think of your own noble family and work with you to fit it into his world making adjustments as necessary, like a
proper DM and not a jerk?
He could say that even though he hasn't thought of everything in the world, I
still cannot invent this new regiment. Why? Because it would mess with his world? Because he is so jealous of his authority that no-one except him can contribute?
What if I say my parents were called Janet and John? Would a DM say, "No! It's
my world, and I decide what the NPCs are called, not you!" Sure, he
could say that, but what kind of jerk would actually prevent the player from making up the names of his own PC's parents?
In your understanding of the default D&D setting, where does a Paladin's magic come from? Do they grant it to themselves, based on the strength of their own resolve? And if so, why would there be specific Orders that such convictions must fall into? Why can't anyone gain magic power, regardless of what they believe, as long as they believe it strongly enough?
They can. Those that do may become paladins
And that's basically what it comes down to. A player who shows up with their own specific Oath interpretation is the equivalent of telling the DM how Oaths work in that world, and that they're definitely up to the individual rather than coming from one of a few specific divine sources. And if the DM doesn't agree with that, because they interpret the text as saying that the Orders correspond to a few specific divine Powers, then it's the player telling the DM that the DM's interpretation of how the world works is wrong.
The Oaths are like laws: you cannot break them. Imagine a law that says 'No smoking'. If you smoke, you've broken the law. If you don't, you have not broken that law.
Now imagine a law which says 'Be nice to people'. Okay, I do my best to be nice to people. One day, I see a beggar and give him a gold piece. Is that nice? Sure. Have I broken the 'be nice' law? No.
But my DM says that I've lost my paladin powers! WTF? Why? "Because you
could have given the beggar 10 gp instead of 1gp, and I'm the DM and what I say goes and how I interpret the text of the law trumps yours!"
That's the kind of DM I'm complaining about!
Meanwhile, there are those who (correctly) see the game mechanics as a metagame rules structure that allows us all to play in our imaginary worlds, but the imagination part is up to us. Just to give you an idea of what we mean, here are three example character ideas for each class, but the fluff is up to you.
Then there are those who (wrongly!) read the exact same words and come to the conclusion that the only
allowed PCs are those three examples per class. That the game mechanics of class (barbarian, bard, cleric) are not only the metagame but also the in-world reality. That a 'fighter' is something that the creatures in the game world can know, as opposed to a thief. That the creatures in the game world can get a microscope and tell the difference between a 'fighter' and a 'thief' by looking.
It's rare I say this, but those guys are role-playing wrong.