I mean the GM is always restricted in how to adjudicate
Yes.
Can you elaborate? I'm curious as to your take on this.
Well, one part of it is what
@Ovinomancer posted:
The only constraint on GMs in D&D is the social contract.
But I think that understates things a bit. After all, in the context of the typically voluntary, leisure activity of RPGing all that ever makes rules binding is an agreement to play the game by the rules. And in the context of some D&D play, as much as any other RPG, there can be a social agreement to be bound by some rules.
As an example, I think in most D&D games, the GM is obliged to accept the PC sheet as a prima facie statement of the PC's mechanical capacities within the game. The GM has some liberty to secretly add to the sheet (eg the PC has a curse or a blessing the player doesn't know about yet) but is not entitled to do so in an arbitrary manner. For instance, the GM isn't normally allowed to make that sort of decision
in the course of working out whether or not a roll to hit succeeds against a target's AC, or in the middle of a resolving
a climb walls roll. And we can explain where the constraint comes from in at least some detail:
making the roll to hit, or to climb walls is something that occurs at the table that, and in that moment of actually rolling the dice and then consulting the relevant material (stat blocks, look-up tables, whatever they might be)
nothing has occurred in the fiction that might make a secret change to the PC and hence to the PC sheet.
@hawkeyefan gives similar examples to the ones I've given, in post 114.n
There are also constraints that relate to the narration of consequences of attempted actions.
@Campbell gave examples in post 117. In a classic AD&D game, for instance, a GM is not generally permitted to adjudicate a failed roll to climb walls as the cliff starting to crumble and collapse on the character - whereas that might be fair game in Burning Wheel! The constraint here is something along the lines of: narrations of failure are expected to be narrations of how the character failed to manifest sufficient skill and/or luck in performing the attempted task, and not as circumstances or the larger environment conspiring against the character. Another example I once posted that caused much outrage on these boards: narrating a failed Diplomacy check as
it starts to rain, and so the crowd can't hear your words and drifts away to find shelter. In standard D&D play that sort of narration of failure is not permitted.
Once we get beyond adjudication to other aspects of the GM's role, like framing and presenting adversity,
@Campbell gives further example of what is out of bounds in standard D&D play. This is harder to state as a rule, but there are clearly widely accepted principles at work.
EDIT:
nature doesn't care one way or the other.
There's a constraint that typically operates on a D&D GM, that does not operate on a Burning Wheel one!