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Term: Triggers Rulings
What is a triggered ability?

A triggered ability is an ability that’s caused by a specified game action. They’re usually written in the
form "When [something happens] -> [effect]." The effect happens just after the specified game action is taken. For example, many characters have triggered abilities that start with "When you flip to this
mode." The effects of these abilities happen immediately after you flip the character to the appropriate
Source: Wizards of the Coast (2018-10-11)    【Link to Ruling】
Tags: Triggers
What happens if more than one triggered ability triggers at the same time?

Triggered abilities always happen one at a time. If all the triggered abilities belong to one player, that
player chooses their order. If both players have triggered abilities that would happen at the same time,
first the player whose turn it is handles all their triggered abilities in any order, then the other player
does the same with their triggered abilities.
Source: Wizards of the Coast (2018-10-11)    【Link to Ruling】
Tags: Triggers
Simultaneous triggers are resolved in the order you choose, but when do you choose the order? Is it when they trigger, or as each resolves? Example: Abilities A, B, and C all trigger simultaneously. Can I resolve A, see the results of that, and then choose whether I want to do B or C next?

Yes. You choose which to use one at a time.
Source: Wizards of the Coast (2019-04-12)    【Link to Ruling】
Tags: Triggers
How do nested triggers resolve? Example: I trigger A, B, and C. B triggers D and E. What order do those resolve in? (Assuming choices, of course).

Assuming you chose B as the first one, you can then choose A, C, D, or E.

Note that Wizards of the Coast changed this with theExpanded Rules 2.0. When a nested trigger occurs, you put any unresolved triggers aside, resolve the new ones, and then go back to the triggers you put aside. So in the example above, it would be, assuming B first: B, then D/E, then A/C.
Source: Wizards of the Coast (2019-04-12)    【Link to Ruling】
Tags: Triggers
If a card with a 'When this battles' trigger (e.g.: Emergency Defense Field) is played on a character after the attacker is declared, will the "When this battles' ability work?
The trigger for all "when this attacks/defends/battles" is at the same time and happens only once as the attacker is declared and tapped. If you use a "when this battles" trigger on a character to then play a "when this battles" upgrade onto that character, the upgrade won't trigger as it's now too late in the attack for those effects to trigger.
Source: Wizards of the Coast (2020-06-10)    【Link to Ruling】
Tags: Emergency Defense Field, Triggers