In context with "The GM Decides"--and (I thought) specifically talking about action resolution--I thought it was clear, but I'll try to be clearer.
Talking specifically about action resolution:
"The GM Decides" is the GM deciding that there is no doubt about the outcome of an action; either it cannot succeed or it cannot fail.
"The Dice Decide" is the outcome is in doubt and the dice determine the outcome.
I don't understand what you mean by "the dice determine the outcome". Can you give an example?
All the examples of RPG mechanics that I'm thinking of at the moment involve
a player declaring an action for his/her PC and then using the dice to find out whether or not the action succeeds. So it is as
@chaochou has said: if the player wins the dice roll
s/he decides what happens (because his/her desired change in the fiction comes true) and if the player loses then the GM decides what happens.
I guess there's one exception I can think of: at least on one interpretation of the relevant rules, in AD&D and Moldvay Basic the GM should roll a reaction roll to establish the starting attitude of NPCs and monsters encountered
whether or not the players have their PCs initiate interaction. That is not a mechanic that gives players any agency - it is a device the GM uses to randomise the framing of the situation. But that can't be an example of what you have in mind precisely because it is not about action resolution. As I just said, it's about framing.
The PCs are in a world. They are not the first characters in that world, nor will they be the last. I try to keep the world consistent and occasionally have things happen that are unrelated to the PCs.
Someone somehwere has probably played a game of D&D where this was not true, but I think it's pretty typical.
But it doesn't answer any of my questions. You're talking about the content of the fiction. I'm asking
how is that fiction authored?
By whom?
And how is that authored ficiton used in subsequent adjudicaitons of declared actions?
If a game is set in the real world, there are going to be things the PCs won't be able to do, and some of those things will be impossible because of the GM's understanding of the real world, which might be different from how the real world objectively works
But now you're just assuming that players dont have agency. From time-to-time I GM games that take place in the "real world" - Cthulhu Dark and most recently Wuthering Heights. The players as much as me get to express views over what can be done in the real world. Eg in one of our Cthulhu Dark sessions the PCs had taken control of a tug boat and the player who knew the most about tug boats told us what could be done with it.
In my games set in non-real worlds - eg my 4e game - the players also help decide what can or can't be done. Eg in that game it was the player of the invoker/wizard who generally took the lead in deciding what was possible to be done with magical effects.
This is why - multiple times upthread now - I have emphasised that
establishing constraints of genre and fictional positioning can be a matter of negotiation and consensus, in which the players exercise their agency as participants in that process. It need not be unilateral GM authority.
And the fact that it can be negotiated is a reason for distinguishing it from
action resolution procedures which, in the traditional RPGs that I play, are not about negotiation but rather involve rolling dice to see whether or not the fiction unfolds as the player is hoping for his/her PC.
It's possible that I'm the only one still in this conversation who separates player agency (the ability to choose what a character does) from narrative authority (the ability to tell the story). The ability to choose--whether to try to swim across the river--is player agency; the ability to describe the result--a current or a monster or angels or a canned leafy green vegetable--is narrative authority. I have played at least one game where narrative authority was not dependent on success in task resolution--so you might fail at a task and have authority to narrate that failure.
As I just posted in reply to
@FrogReaver, the definition of
player agency that you posit here is uninteresting because in every RPG players have it. It's not something that varies.
@chaochou has made it crystal clear that by
player agency he means
the ability of the player to change the state of the shared fiction. Given that, in a traditional RPG, the way a player changes the shared fiction is by declaring actions for his/her PC and then having those resolve, the connection between player agency and action resolution procedures is not coincidental.
If a player can't change the shared fiction; if all s/he can do is prompt the GM to make such changes by describing what it is that his/her PC tries to do; then what is the role of the player in the game?