What it says in the title.
When you're looking at some silly build like the Sorc 5/Warlock 5/Fighter 2, do you try to justify the multiclass in-game, or do you prefer to ignore the fiction and concentrate on eldritch blasting all the things?
Relevant comic.
Let me turn this around:
I have never played in a world where the characters are aware that they have classes,what they are, and use them to define themselves and others. Trying to say that class - single or multi - is an in-game construct is patently false.
Now, every character interacts with the narrative, with what's going on. Some of these are free-form, non-mechanical interactions. Some of these are mechanical interactions within the system.
So if my character is brave, what class am I? No information, maaaaaybe we could assume you have a good Wisdom saving throw since that's how the system would support it when/if comes up. If my character can fire a bow well, what class am I? Well, that has some more mechanical expression so we can narrow to one of several classes. Or a combonation of them. Or perhaps them and something else because we haven't gotten a full read on the character. Is that archer woodsman with nature spells a ranger, or maybe a land druid / rogue (scout)? Or maybe a fighter / cleric (nature) with an appropriate background.
Singleclassing and multiclassing are just ways of picking the mechanical expression that best matches how your character interacts with the system part of the narrative. Multiclassing is a great tool when you aren't willing to limit your character vision to a set of pre-defined containers, instead mixing and matchng to better be able to represent in the narrative.
Be willing to play whatever is needed in order to realize the vision you have of your character, and that includes how they interact with the narrative. If you want to play a dashing swashbuckler, make them able to show themselves as both dashing and a swashbuckler - or any other combo. Don't let yourself be pidgeonholed, take the race, background and class(es) that best represent what you are trying to show.
Night and day with you. Yes, class is not an in-game construct, but classes are more than just ability packages. In-game "fighter" is meaningless, as it could include rogues, barbarians or rangers, but on the other hand Wizard clearly is an in-game thing,and a very specific in game thing. Sorcerer is also a clear in-game thing, even if most of them wouldn't even have a word for it, but sorcerers can clearly see they are indeed not wizards, or at least as soon as they cross paths with one. Paladin is another in-game reality, even if in-game avengers, wardens and devotion paladins don't recognize each other as the same thing, they clearly wouldn't confuse themselves with any run of the mill warrior.
Yet only one of these kinds will leave a spellbook behind for your wizard to collect.There are plenty of NPC casters who are "wizards" or "sorcerers" or whatever but don't follow PC rules.
A paladin is a paladin, all of them might be champions of the gods, but only a paladin is a paladin.Second, "paladin" as an in-game concept would strongly depend on the setting. A blessed champion of a god, sure - but there's plenty of ways to build those. Could a religious monk not consider his abilities to be blessings from the gods? A barbarian (zealot) falling into a holy rage? Maybe that's just how a cleric (war) describes herself. And that's before multiclassing or reskinning.
Yet only a true rogue knows Thieves cant. Like I said, not all classes are really reflected as in-game things, but some of them are clearly different things in-universe.I've played a character who would be considered a "rogue" or "thief" in world but that was just through background, RP and a good dex - they didn't have any levels of Rogue. Class doesn't define how you present.
One of the best parts of 4e was the explicit permission to reskin. If your barbarian (totem) takes on more aspects of a bear, there's no mechanical reason not to describe getting shaggy with hair and some werebear blood coming to the fore. Same for anything else - classes are bundles of ways to interact with the mechanics of the game, not fetters to insist on how your character must be seen in the game.
A paladin is a paladin, all of them might be champions of the gods, but only a paladin is a paladin.
Yet only a true rogue knows Thieves cant. Like I said, not all classes are really reflected as in-game things, but some of them are clearly different things in-universe.
To each their own, to me it was one of the hardest parts to stomach. The problem with refluffing is that eventually communication breaks down. We cannot communicate if one of us randomly decides words start meaning completely different things. Just like in-game reality is a shared reality, you might prefer to ignore default fluff, but that doesn't mean the rest of the group will. Even then, default fluff shapes the new fluff to a certain extent -via mechanics-.