How does deciding on the DC work differently within indie games?
Are players part of the process of deciding or is it GM decides/notes?
@Manbearcat provided an excellent survey of many of the games that commonly get floated in these discussions. I can supply a few other indie games.
Fate (Core, Accelerated, Condensed)
Fate is more traditional when it comes to DCs in that the GM often sets the difficulty, though this is done in one of two ways: rolling opposition or setting difficulty. If the PCs are facing opposition, particularly from NPCs, the GM may roll the attack or defense of the opposition. The success of the attack depends on the outcome of the defense roll (e.g., fail, tie, success, etc.). Moreover, in Fate opposition rolls can be used in combat or even social scenes (e.g., Provoke attack vs. Will defense). Or much like in 4e D&D, the GM may decide that a series of skill challenges are required.
For setting the difficulty, there is a DC scale, with associated adjectives (e.g., poor, -1; mediocre, +0; average, +1; fair, +2; etc.). Fate provides GM advice for when to use low, medium, and high difficulties in terms of what it means for the PCs: e.g., use low to give them a chance to show off,
medium to provide tension but not overwhelm them, or
high to emphasize dire or unusual circumstances. There is also the general advice that if you can think of one reason why a task is tough, the GM should generally choose Fair (+2) and add +2 for every additional reason.
This also does not include the possibility for the GM to invoke/compel aspects that are in play: e.g., PCs' aspects, NPCs' aspects, scene aspects, etc. This can affect the opposition or DC.
The players may roll to beat this DC using their Approaches, Skills, Rated Aspects, relevant Stunts, etc. If they fail, players can spend fate points to invoke the relevant aspects from their character for a +2 per invoked aspect or to re-roll.
Cortex (Prime)
In the basic version of Cortex Prime, the GM assembles an opposition pool. This is typically two dice of the same type, based on the perceived difficulty of the situation in the fiction - e.g., Very Easy (2d4); Easy (2d6); Challenging (2d8); Hard (2d10); Very Hard (2d12) - which is then added together to set the difficulty. So although the GM does determine the general rating of the dice, there is some randomness as a result of the die roll. All rolls are openly rolled, including the GM's.
Players then assemble their own character's dice pool. Again, it's add (typically) the two highest from rolling the player's dice pool to exceed the difficulty rolled by the GM.
Players assemble their pool through selecting one rated die from each of their traits (i.e., Prime Sets) plus any secondary traits that may be relevant. Cortex assumes, however, that players will have at least
three prime sets: i.e., Distinctions (similar to Fate's character aspects) plus two to three more (e.g., Attributes, Skills, Roles, Affiliations, Relationships, Values, Resources, etc.). These have to be based on and relevant to the fiction. Because of the game's toolkit approach, these prime sets will vary between versions.
Moreover, since Cortex is highly modular, there are a fair number of mods about how to change how difficulty is set: e.g., Doom Pool.