Planning a Character's "Arc" in Practice

This thread isn't meant to start angry arguments about whether or not this is good play, or the correct way to play, or a sign of some malaise in the hobby or whatever. Those arguments are relitigated every few weeks or so when someone asks a question about backstory and always go the same ways.

Every so often when there's discussion of how people run their games there's allusions to players "planning out a PC's arc and informing the GM of it"; it's always contentious and generally invites what to me feel like massive overreactions from commenters about it being bad play.

What I want to ask is the GMs and players who do this what it actually entails, how it plays out in the course of a campaign and just generally what your experiences of it are *precisely because* it's not a form of play I've experienced. Or, is this the internet being what it is and bad expression of an idea is making something routine seem exceptional and inviting bad-faith interpretations of it?

I feel a lot of fiction-forward systems see character progression as deepening of narrative role, in a sense; playbook advancements strengthening whichever trope or role the playbook represents, for example. I also see systems like World of Darkness encouraging you to have ideals, or goals, or ambitions and having those shape your character and give themes for the GM to include (in a *Mage* game if your Obsession is about seeking immortality, then the GM can use that to shape the encounters and events you'll see along the way, as a basic example).

Or, even less mechanically shaped, am I "planning an arc" if I say at session zero "my character is someone who's never wanted for anything and now has to deal with losing all their privilege", which is describing how I'll shape my RP throughout the game and potentially suggesting to the GM I'd be interested in stuff about class consciousness and so on.

I think in the end I've got a few main questions here which I'm asking not to give me food to mock people who play in different ways to my table but to try and learn how other tables run games in good faith.

1. In practice, as a PC or DM, what does "planning an arc" entail? 2. Does this even happen in statistically significant amounts or is it a canard invented by people trying to mischaracterise fiction-forward or character-driven RPGs and do down alternative styles of play?

Because if you listen to some people it's "each session the player tells the GM what will happen to their character" which sounds a gross exaggeration, and if you listen to others it's anything more than having a purely emergent characterisation revealed through play.