Canon is the past in the play. It establishes facts which have gone before. It doesn’t mean it can’t be retconned afterwards but for the most part it is immutable (or at least it should have immutable facts and mutable descriptors, the latter of which may be changed due to Context.
Context is the present in the play. It represents experiences more than facts and, due to the way we lay down memories, is probably more immediately mutable than Canon though if left, it can be harder to change because it forms part of our experience memory and not just written Canon (which is essentially the basis of ‘show, don’t tell’ – if someone visualises the experience they will have deeper connection to it rather than just having the story related to them).
Accepting is where you acknowledge and agree with the contributions from others to the Context settling it as Canon. This is most easily visualised when you try to reconcile the order of events taking place in a system which does not have a strict action-point (or similar) mechanic. In systems with Action Points or Strike Ranks, everything happens according to the time. To move the story along, it can be necessary to fudge the timings, e.g.
Zakary and Carey are two of the contestants in the Garhound contest and Zakary has caught up with Carey who is descending from the Storm Apple tree with a ripe Storm Apple. Though they are perhaps 4-5 SRs apart, the GM permits Zakary to assault Carey and steal the Storm Apple. Why? Because it will make for further flavour and discussion in the game. Who loses out? An NPC who wasn’t doing well in the race anyway.
In a more narrative game, it can mean just the end of discussing a scene. Not everyone needs to be entirely happy with the outcome, but there has to be a sense of mutual consent so that the game can continue.
Returning – negates the contribution to Context but also provides options which can be considered. This is reserved for when there is disagreement within the group as to the interpretation and outcome of a scene. This is most common when there has been a GM fiat about a character and the player disagrees with it. It behooves the player in this circumstance to not only make his grievance known but also to provide a suitable alternative. This may, dependiing on the detail, be returned to the player again or returned to the group as a whole for a better resolution, e.g.
It would have been entirely appropriate for other players to dispute the conflict between Zakary and Carey because there was an immediate gain for Zakary. Though he had no chance of winning the race, Storm Apples are a potent magical item. What convinced them ultimately was that it would have little effect on their characters and in no small part would give Carey, an annoying NPC, a harder time of it. They had agreement and the option was not returned.
It’s much harder to retcon against a dice roll because the randomising element is a central tenet of roleplaying. We use them to make the decisions for us and if we stop ‘trusting’ their decisions, we may as well switch to a fully narrative process (no bad thing in of itself).